message: Clean up the CI install script.
diff: set -ex
  -if [ "$CLIPPY" = "yes" ]; then
  - rustup component add clippy-preview
  -fi
  -
  if [ "$DOCS" = "yes" ]; then
  - cargo install mdbook --no-default-features --force
  - cargo install mdbook-linkcheck --force
  + cargo install mdbook --no-default-features
  + cargo install mdbook-linkcheck
  fi
  if [ "$TRAVIS_OS_NAME" = "osx" ] && [ "$GNUPLOT" = "yes" ]; then
  brew unlink python@2 # because we're installing python3 and they both want to install stuff under /usr/local/Frameworks/Python.framework/
  brew install gnuplot
  fi
  -
  -if [ "$RUSTFMT" = "yes" ]; then
  - rustup component add rustfmt-preview
  -fi
  