Use Starlark version of 'http_archive' for Bazel >= 0.20 compatibility

[sgbalogh]

Hi,

I ran into an issue while trying to build this project using Bazel 0.20.0:

$ bazel build //:remote_client
Starting local Bazel server and connecting to it...
INFO: Invocation ID: d32c4b6a-9707-4801-8bea-4a1c8691ad93
ERROR: Analysis of target '//:remote_client' failed; build aborted: no such package '@googleapis//': The native new_http_archive rule is deprecated. load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive") for a drop-in replacement.
Use --incompatible_remove_native_http_archive=false to temporarily continue using the native rule.
INFO: Elapsed time: 6.744s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (8 packages loaded, 23 targets configured)
    Fetching @googleapis; fetching

It appears that the native 'http_archive' functions have been deprecated as of Bazel 0.20. This patch imports the replacements from the Starlark extensions. See: bazelbuild/bazel#6570 for reference.

Making these changes allowed the build to complete.

Thanks!
-S