Log Message:
Update ruby-sass to 3.1.16.

3.1.16

* Fix some bugs in sass-convert selector parsing when converting from CSS.
* Substantially improve compilation performance on Ruby 1.8.
* Support the @-moz-document directive¡Çs non-standard url-prefix and domain
  function syntax.
* Support the @supports directive.
* Fix a performance issue when using /*! */ comments with the Rails asset
  pipeline.
* Support -moz-element.
* Properly handle empty lists in sass-convert.
* Move from FSSM to Listen for file-system monitoring.