message: Avoid caching the target directory during CI
  The travis caches are currently between 5 and 10Gb and the build spends a lot of time (about 6 mins x 2) downloading and unpacking before the build and packing and uploading afterwards.
diff: language: rust
  -cache: cargo
  +cache:
  + directories:
  + - $HOME/.cargo
  +
  + # Don't cache the target directory. It gets rather large, and the
  + # unpacking of this ends up taking a rather long time!
  + # - $TRAVIS_BUILD_DIR/target
  rust:
  - 1.36.0
  