Originally, the Travis CI configuration had an overridden base queryset

used to limit results to GitHub repositories, in order to help

administrators select only compatible repositories.
This has since broken, as get_queryset() was overridden and no longer

utilized the base queryset, falling back to showing all repositories

again.
This change fixes that by overriding get_queryset() and filtering the

parent method's result, once again enabling the filtering.
Unit tests are added to ensure this doesn't regress.

Unit tests pass.
Verified that only GitHub repositories appeared when viewing the form.