message: [travis] Enable cache for $HOME/.cargo/bin/
  This should accelerate the `cargo install mdbook` step under `install`,
  which is currently called on all linux+nightly setups.
diff: @@ -6,6 +6,10 @@ dist: trusty
  git:
  depth: 1
  +cache:
  + directories:
  + - $HOME/.cargo/bin/
  +
  matrix:
  include:
  - env: TARGET=x86_64-unknown-linux-gnu
  @@ -22,7 +26,7 @@ matrix:
  ALT=i686-unknown-linux-gnu
  rust: nightly
  install:
  - - cargo install mdbook
  + - mdbook --help || cargo install mdbook --force
  script:
  - cargo test
  - cargo doc --no-deps
  