Auto merge of #4481 - behnam:travis, r=alexcrichton
authorbors <bors@rust-lang.org>
Fri, 22 Sep 2017 14:34:26 +0000 (14:34 +0000)
committerbors <bors@rust-lang.org>
Fri, 22 Sep 2017 14:34:26 +0000 (14:34 +0000)
[travis] Enable cache for $HOME/.cargo

This should accelerate the `cargo install mdbook` step under `install`,
which is currently called on all linux+nightly setups.

https://github.com/rust-lang/cargo/blob/36ddeff03a31f710095cfbe49287503bb60e294d/.travis.yml#L24-L25


Trivial merge