Merge branch 'gitlab-ci' into 'master'

GitLab-CI support

What was done:
* Converted `.travis-ci.yml` to the equivalent in `.gitlab-ci.yml` and added additional rubies.
* JRuby support is removed as we don't use and a dependent gem removed the support for it: https://github.com/jgarber/redcloth/issues/31#issuecomment-252954871
* Fixed `README.pod` fixture to use a valid markup
* Fixed `README.pod.html` to include updated expected render.

See merge request !1