Auto merge of #4481 - behnam:travis, r=alexcrichton
Auto merge of #4481 - behnam:travis, r=alexcrichton

[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

vojtechkral/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo,vojtechkral/cargo,eminence/cargo,tsurai/cargo,rust-lang/cargo,nipunn1313/cargo,tsurai/cargo,vojtechkral/cargo,vojtechkral/cargo,achanda/cargo,rust-lang/cargo,tshepang/cargo,tshepang/cargo,tsurai/cargo,rustyhorde/cargo,tsurai/cargo,achanda/cargo,nipunn1313/cargo,rustyhorde/cargo,nipunn1313/cargo,rustyhorde/cargo,eminence/cargo,tshepang/cargo,rust-lang/cargo,tsurai/cargo,achanda/cargo,achanda/cargo,eminence/cargo,tshepang/cargo,rust-lang/cargo