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`.

.gitignore

@@ -1,8 +1,11 @@
 # Ignore docs files
 _gh_pages
 _site
+
+# Ignore ruby files
 .ruby-version
-Gemfile.lock
+.bundle
+vendor/cache
 
 # Numerous always-ignore extensions
 *.diff
@@ -41,6 +44,9 @@ validation-status.json
 # SCSS-Lint
 scss-lint-report.xml
 
+# grunt-contrib-sass cache
+.sass-cache
+
 # Folders to ignore
 bower_components
 node_modules