Removes duplicate from Gemfile and arranges groups

There were duplicate entries in the Gemfile that were causing warnings.
The duplicates were removed and the gems were placed in their correct
groups.
Merge pull request #65 from 18F/feature/remove_duplicate_entries_from_gemfile

Removes duplicate from Gemfile and arranges groups
Fixes bug in category#index view and adds tests

There is a bug in the category#index view since it is expecting an
article object to exist. It seems it was intended to only display
categories with published articles so I added the code needed for that
(and tests). I also moved dotenv-rails in the Gemfile so that it is
included in the production env.