Add comments explaining the rustfmt installation dance.

.travis.yml

@@ -17,13 +17,20 @@ install:
     - pip3 install --user --upgrade mypy flake8
     - mypy --version
 before_script:
+    # If an old version of rustfmt from cargo is already installed, uninstall
+    # it, since it can prevent the installation of the new version from rustup.
     - cargo uninstall rustfmt || true
     - cargo install --list
+    # If we're testing beta or nightly, we still need to install the stable
+    # toolchain so that we can run the stable version of rustfmt.
     - rustup toolchain install stable
+    # Install the stable version of rustfmt.
     - rustup component add --toolchain=stable rustfmt-preview
     - rustup component list --toolchain=stable
     - rustup show
     - rustfmt +stable --version || echo fail
+    # Sometimes the component isn't actually ready after being installed, and
+    # rustup update makes it ready.
     - rustup update
     - rustfmt +stable --version
 script: ./test-all.sh