Remove ruby 2.1 from travis, because rails 5 doesn't support it anymore
Remove ruby 2.1 from travis, because rails 5 doesn't support it anymore

The gem still supports ruby 2.1 (without rails or with rails 4.2), but I
can't run tests for it on travis.

diaspora/diaspora_federation,SuperTux88/diaspora_federation,diaspora/diaspora_federation,SuperTux88/diaspora_federation