Fail to build runtime `0.3.0-alpha.6` from crates.io

[oxalica]

Steps to reproduce:

1. cargo init
2. Add runtime = "=0.3.0-alpha.6" as dependency.
3. cargo build, and it fails with the following log:

   ...
   Compiling futures-timer v0.2.1
error[E0277]: the trait bound `ext::TimeoutStream<S>: futures_core::stream::Stream` is not satisfied
   --> /home/oxa/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-timer-0.2.1/src/ext.rs:170:9
    |
170 | impl<S> TryStream for TimeoutStream<S>
    |         ^^^^^^^^^ the trait `futures_core::stream::Stream` is not implemented for `ext::TimeoutStream<S>`
    |
    = note: required because of the requirements on the impl of `futures_core::stream::private_try_stream::Sealed` for `ext::TimeoutStream<S>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: Could not compile `futures-timer`.
warning: build failed, waiting for other jobs to finish...
error: build failed

My rustc version is rustc 1.38.0-nightly (8a58268b5 2019-07-31).

Current master compiles well with futures-timer bumped.

Seems simply bumping and publishing 0.3.0-alpha.7 will fix it.

[taiki-e]

Published runtime 0.3.0-alpha.7 which fixes this issue.