Improve CI caching

We weren't making very effective use of the cache before, leading to
10m+ builds, as `cargo install --force` always busts the cache.

Author: benesch

.travis.yml

@@ -24,7 +24,18 @@ rust:
 
 before_script:
   - pip install 'travis-cargo<0.2' --user && export PATH=$HOME/.local/bin:$PATH
-  - cargo install --force cargo-travis && export PATH=$HOME/.cargo/bin:$PATH
+  - export PATH=$HOME/.cargo/bin:$PATH
+  # `cargo install` fails if the specified binary is already installed, and
+  # doesn't yet support a `--if-not-installed` option [0], so for now assume
+  # failures mean the package is already installed. If installation truly
+  # failed, the build will fail later anyway, when we try to use the installed
+  # binary. Note that `cargo install --force` is not a solution, as it always
+  # rebuilds from scratch, ignoring the cache entirely.
+  #
+  # [0]: https://github.com/rust-lang/cargo/issues/2082
+  - cargo install cargo-update || echo "cargo-update already installed"
+  - cargo install cargo-travis || echo "cargo-travis already installed"
+  - cargo install-update -a  # updates cargo-travis, if the cached version is outdated
   - rustup component add clippy rustfmt
 
 script: