gitlab-ci

Gem version updates

### Bump rails, jquery-rails, sprockets, sass-rails versions

Addresses security advisories:

- https://groups.google.com/forum/#!topic/ruby-security-ann/XIZPbobuwaY
- https://groups.google.com/forum/#!topic/ruby-security-ann/gcUbICUmKMc

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### Bump coveralls version

Addresses security issues in its rest-client dependencies:

- https://github.com/rest-client/rest-client/issues/369
- http://www.osvdb.org/show/osvdb/117461

Closes #214

See merge request !194