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/gemfiles/core.gemfile

@@ -0,0 +1,7 @@
+# Ruby Gems for the 'core' set of tests
+# Run `grunt update-gemfile-lock` to update to the latest compatible versions
+
+source 'https://rubygems.org'
+
+gem 'sass', '~> 3.4.9'
+gem 'scss-lint', '~> 0.31'

test-infra/gemfiles/core.gemfile.lock

@@ -0,0 +1,15 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    rainbow (2.0.0)
+    sass (3.4.9)
+    scss-lint (0.31.0)
+      rainbow (~> 2.0)
+      sass (~> 3.4.1)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  sass (~> 3.4.9)
+  scss-lint (~> 0.31)