Persist group category setting back to the group (via description tag)

## Take note before merging
- You might like to hold off merging this pull request for:
  - Bug in Google Admin's Group edit form: it doesn't seem to respect carriage returns, until a pull request for modifying group's description (not config hash) via gaps is implemented.
  - Until a rake task to migrate existing groups to use the config hash is implemented.
- Updated .ruby-version from 2.1 to 2.1.5
- Added RSpec
- Until we decide to add test environments with mongodb, I've stubbed out some specs related to saving to mongodb (line 50 in group_spec.rb).
Nice catch! I changed this before I realized the bug in Google Admin's edit form, then I forgot about it. My latest commit d3270a3 should take care of it. 

Let me know if there's a more straightforward way or if it's unnecessary, I'll revert it.