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
Add various dev tools and update rspec

Added better_errors, binding of caller, and pry-nav. Also upgrades
rspec to 2.99 and fixes some deprecations for v3.