Added Rails 4.2.x support.
Added `null: false` to model timestamp columns.

- This must be specified in Rails 4.2.x.
Refactored language statistics to accept a language instance rather
than ID.

- More often than not, it's an instance of a particular language we
  want to deal with, not the ID.
- These changes require that a language instance always be passed in
  where as, previously, both a language instance and ID could be used.
  This removes the ambiquity.