Install rustfmt as a separate Travis install step.

- Add a check-rustfmt.sh script which checks if the right version of
  rustfmt is installed.
- Run check-rustfmt.sh --install as an install step under travis_wait.

This is to work around the issue where cargo takes forever to build
rustfmt, causing Travis to terminate the build because it hasn't
produced any output for 10 minutes.

.travis.yml

@@ -10,6 +10,7 @@ addons:
             - python3-pip
 install:
     - pip3 install --user --upgrade mypy flake8
+    - travis_wait ./check-rustfmt.sh --install
 script: ./test-all.sh
 cache:
     cargo: true