Downgrade tokio deps to 0.2 (deadpool)

[pjenvey]

deadpool's recently upgraded to the latest tokio (0.3 then 1.0) which causes incompatibilities w/ actix-web's tokio 0.2 executor.

Somehow it managed to run ok under this executor until it attempted to use a tokio timer (which began being used for #973). The timer issue errors out with:

thread 'actix-rt:worker:12' panicked at 'there is no timer running, must be called from the context of Tokio runtime or `rt` is not enabled', /home/pjenvey/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.3.6/src/time/driver/handle.rs:74:13
stack backtrace:
   0:     0x55cebe59b6d0 - std::backtrace_rs::backtrace::libunwind::trace::h04d12fdcddff82aa
                               at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/../../backtrace/src/backtrace/libunwind.rs:100:5
   1:     0x55cebe59b6d0 - std::backtrace_rs::backtrace::trace_unsynchronized::h1459b974b6fbe5e1
                               at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x55cebe59b6d0 - std::sys_common::backtrace::_print_fmt::h9b8396a669123d95
                               at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x55cebe59b6d0 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::he009dcaaa75eed60
                               at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/sys_common/backtrace.rs:46:22
   4:     0x55cebe5c026c - core::fmt::write::h77b4746b0dea1dd3
                               at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/core/src/fmt/mod.rs:1078:17
   5:     0x55cebe593b52 - std::io::Write::write_fmt::heb7e50902e98831c
                               at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/io/mod.rs:1518:15
   6:     0x55cebe59df45 - std::sys_common::backtrace::_print::h2d880c9e69a21be9
                               at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/sys_common/backtrace.rs:49:5
   7:     0x55cebe59df45 - std::sys_common::backtrace::print::h5f02b1bb49f36879
                               at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/sys_common/backtrace.rs:36:9
   8:     0x55cebe59df45 - std::panicking::default_hook::{{closure}}::h658e288a7a809b29
                               at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/panicking.rs:208:50
   9:     0x55cebe59dbe8 - std::panicking::default_hook::hb52d73f0da9a4bb8
                               at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/panicking.rs:227:9
  10:     0x55cebcefb256 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h5e75b4c0967974a5
                               at /media/ext/pjenvey/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:1321:9
  11:     0x55cebcef9fff - <sentry_panic::PanicIntegration as sentry_core::integration::Integration>::setup::{{closure}}::{{closure}}::h200563c26d072247
                               at /home/pjenvey/.cargo/registry/src/github.com-1ecc6299db9ec823/sentry-panic-0.19.1/src/lib.rs:59:17
  12:     0x55cebe59e706 - std::panicking::rust_panic_with_hook::hfe7e1c684e3e6462
                               at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/panicking.rs:597:17
  13:     0x55cebd4d480e - std::panicking::begin_panic::{{closure}}::he24309067a3f8fb1
                               at /media/ext/pjenvey/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:522:9
  14:     0x55cebd4d87a9 - std::sys_common::backtrace::__rust_end_short_backtrace::h42e1d357b8612855
                               at /media/ext/pjenvey/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys_common/backtrace.rs:141:18
  15:     0x55cebd4d473b - std::panicking::begin_panic::h55098292cdf83d4b
                               at /media/ext/pjenvey/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:521:12
  16:     0x55cebd4d91b4 - tokio::time::driver::handle::Handle::current::hf38d47c909a2f3b6
                               at /home/pjenvey/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.3.6/src/time/driver/handle.rs:74:13
  17:     0x55cebd4da303 - tokio::time::driver::sleep::Sleep::new_timeout::h462ca9907077e221
                               at /home/pjenvey/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.3.6/src/time/driver/sleep.rs:74:22
  18:     0x55cebcd77405 - tokio::time::timeout::timeout::h4a84f9f736156adf
                               at /home/pjenvey/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.3.6/src/time/timeout.rs:52:17
  19:     0x55cebcc2a3a5 - deadpool::managed::apply_timeout::{{closure}}::h612bac31a54c878d
                               at /home/pjenvey/.cargo/registry/src/github.com-1ecc6299db9ec823/deadpool-0.6.0/src/managed/mod.rs:309:33

I'm not sure if there's any compatibility layer to handle this, so a quick fix is to downgrade (and ensure our deps are stuck on the older tokio until actix-web upgrades).

[jrconlin]

I think the compatibility was to enable rt, which we're doing. Downgrading is probably the best option now.