build: fix bindgen warnings breaking TravisCI

When rustfmt is missing, bindgen puts anything on one line. Any warnings then
dump so many logs that TravisCI is overwhelmed. The fix is to add rustfmt to the
environment.

See rust-lang/rust-bindgen#1600

.travis.yml

@@ -15,6 +15,9 @@ addons:
     packages:
       - build-essential
 before_install:
+  # Without rustfmt, bindgen puts everything on one line and any warnings dump so many logs they break Travis
+  # See https://github.com/rust-lang/rust-bindgen/issues/1600
+  - rustup component add rustfmt
   - if [[ $TRAVIS_OS_NAME == 'linux' ]]; then ./.travis/install_linux.sh; fi
   - if [[ $TRAVIS_OS_NAME == 'osx' ]]; then brew update; fi
   - if [[ $TRAVIS_OS_NAME == 'osx' ]]; then brew install yasm; fi