Split out violations into separate table

  * Makes `Violation` inherit from `ActiveRecord::Base` and adds associated
    table
  * Adds `has_many :violations` association to `Build`
  * Updates creation of `violations` in `style_guide`'s
  * Adds `violation` factory

EDIT: this PR also renames the column `builds.violations` to `builds.violations_archive` allowing us to migrate the historical violations at a later date while making sure we're not still using this column in production.

https://trello.com/c/OLdRI0mO/336-split-out-violations-into-separate-table
Interesting! I disagree it's a smell. Along the same lines would you add model validations to join tables? Such as Memberships? https://github.com/thoughtbot/hound/blob/master/app/models/membership.rb#L2