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).
Hm, should this be a string `'category'` rather than a symbol?