Compile with Ruby Sass or libsass.

* Separate configs for libsass and sass.
* Sass compiler selected based on `process.env.TWBS_SASS`.
* Travis:
  * Use Gemfile to manage ruby dependencies.
  * Run core tests with both Sass compilers.
  * Only install/cache ruby gems required by the test subset.
* Grunt: `update-gemfile-lock` task a la `update-shrinkwrap`.

test-infra/S3Cachefile.json

@@ -5,8 +5,8 @@
         "generate": "./uncached-npm-install.sh"
     },
     "rubygems": {
-        "key": "../pseudo_Gemfile.lock",
-        "cache": "$GEMDIR",
-        "generate": "gem install -N scss-lint -v $SCSS_LINT_VERSION && gem install -N jekyll -v $JEKYLL_VERSION && gem install -N rouge -v $ROUGE_VERSION"
+        "key": "$BUNDLE_GEMFILE",
+        "cache": "../vendor/cache",
+        "generate": "cd .. ; bundle install --path=\"`pwd`/vendor/cache\""
     }
 }