Fix a few sorting issues: * default Category sorting to "position ASC", same as with the Forum model * allow forums "position" to be mass-assigned
Add a test for forum_params in Admin::ForumsController
Removed the default scope from Forem::Category and added a by_position scope instead.
Support Ruby 2.1.5 + Remove 1.9.3 and jruby from Travis

1.9.3 is unsupported by Ruby Core.

JRuby constantly errors on Travis, and although we'll be keeping support for it I want the errors to go away.