Update futures requirement to 0.2.0

[dependabot-preview]

Updates the requirements on futures to permit the latest version.

Changelog

Sourced from futures's changelog.

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.
• Unify the Canceled type of the sync and unsync modules.
• Deprecate the boxed methods. These methods have caused more confusion than
  they've solved historically, so it's recommended to use a local extension
  trait or a local helper instead of the trait-based methods.
• Deprecate the Stream::merge method as it's less ergonomic than select.
• Add oneshot::Sender::is_canceled to test if a oneshot is canceled off a
  task.
• Deprecates UnboundedSender::send in favor of a method named unbounded_send
  to avoid a conflict with Sink::send.
• Deprecate the stream::iter function in favor of an stream::iter_ok adaptor
  to avoid the need to deal with Result manually.
• Add an inspect function to the Future and Stream traits along the lines
  of Iterator::inspect

... (truncated)

Commits

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[dependabot-preview]

Superseded by #28.