Update futures requirement to 0.2

[dependabot-preview]

Updates the requirements on futures to permit the latest version.

Changelog

Sourced from futures's changelog.

0.2.1 - 2018-4-19

• Add the futures-stable crate for working with immovable futures.
• Add async/await support behind the nightly feature gate.
• Add Stream::for_each_concurrent.

0.2.0 - 2018-4-6

• Separate out the futures crate into a number of separate crates which
  are reexported by a single facade.
• Add a &mut task::Context parameter to poll methods. This type includes
  access to a wakeup callback, task-local-data, and a default executor.
• Refactor the Sink trait.

0.1.17 - 2017-10-31

• Add a close method on sink::Wait
• Undeprecate stream::iter as stream::iter_result
• Improve performance of wait-related methods
• Tweak buffered sinks with a 0 capacity to forward directly to the underlying
  sink.
• Add FromIterator implementation for FuturesOrdered and FuturesUnordered.

0.1.16 - 2017-09-15

• A prelude module has been added to glob import from and pick up a whole
  bunch of useful types
• sync::mpsc::Sender::poll_ready has been added as an API
• sync::mpsc::Sender::try_send has been added as an API

0.1.15 - 2017-08-24

• Improve performance of BiLock methods
• Implement Clone for FutureResult
• Forward Stream trait through SinkMapErr
• Add stream::futures_ordered next to futures_unordered
• Reimplement Stream::buffered on top of stream::futures_ordered (much more
  efficient at scale).
• Add a with_notify function for abstractions which previously required
  UnparkEvent.
• Add get_ref/get_mut/into_inner functions for stream take/skip methods
• Add a Clone implementation for SharedItem and SharedError
• Add a mpsc::spawn function to spawn a Stream into an Executor
• Add a reunite function for BiLock and the split stream/sink types to
  rejoin two halves and reclaim the original item.
• Add stream::poll_fn to behave similarly to future::poll_fn
• Add Sink::with_flat_map like Iterator::flat_map
• Bump the minimum Rust version to 1.13.0
• Expose AtomicTask in the public API for managing synchronization around task
  notifications.

... (truncated)

Commits

• See full diff in compare view

Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting @dependabot rebase.

---

Note: This repo was added to Dependabot recently, so you'll receive a maximum of 5 PRs for your first few update runs. Once an update run creates fewer than 5 PRs we'll remove that limit.

You can always request more updates by clicking Bump now in your Dependabot dashboard.

Dependabot commands and options

You can trigger Dependabot actions by commenting on this PR:

• @dependabot rebase will rebase this PR
• @dependabot ignore this [patch|minor|major] version will close this PR and stop Dependabot creating any more for this minor/major version (unless you reopen the PR or upgrade to it yourself)
• @dependabot ignore this dependency will close this PR and stop Dependabot creating any more for this dependency (unless you reopen the PR or upgrade to it yourself)
• @dependabot use (this|these) label[s] will set the current labels as the default for future PRs for this repo and language
• @dependabot use (this|these) reviewer[s] will set the current reviewers as the default for future PRs for this repo and language
• @dependabot use (this|these) assignee[s] will set the current assignees as the default for future PRs for this repo and language

Additionally, you can set the following in your Dependabot dashboard:

• Update frequency (including time of day and day of week)
• Automerge options (never/patch/minor, and dev/runtime dependencies)
• Pull request limits (per update run and/or open at any time)
• Out-of-range updates (receive only lockfile updates, if desired)
• Security updates (receive only security updates, if desired)

Finally, you can contact us by mentioning @dependabot.

[dependabot-preview]

OK, I won't notify you again about this release, but will get in touch when a new version is available. If you'd rather skip all updates until the next major or minor version, let me know by commenting @dependabot ignore this major version or @dependabot ignore this minor version.

If you change your mind, just re-open this PR and I'll resolve any conflicts on it.