Improve Travis CI cache

.travis.yml

@@ -22,8 +22,10 @@ env:
     - APM_TEST_PACKAGES="language-marko language-html-swig language-svg"
 
 cache:
-    - pip
+    - pip: true
+    - bundler: true
     - directories:
+        - vendor/bundle # gems are installed here, https://docs.travis-ci.com/user/languages/ruby/#Dependency-Management
         - node_modules
         - $HOME/.atom
         - $HOME/.stack
@@ -56,9 +58,9 @@ before_install:
   # CoffeeScript
   - npm install coffee-formatter
   # Haskell
-  - brew install haskell-stack
-  - stack setup
-  - stack install stylish-haskell
+ - brew install haskell-stack
+ - stack setup
+ - stack install stylish-haskell
   # Elm
   - curl -L -o /tmp/elm-format.tgz https://github.com/avh4/elm-format/releases/download/0.1-alpha2/elm-format-0.1-alpha2-mac-x64.tgz
   - tar xvzf /tmp/elm-format.tgz -C /usr/local/bin