Update error messages to match ActiveModel formatting.

Hi there!  First, thanks for the awesome gem, it's great.  There hasn't been a good enum gem for some time, but this seems to have all the bells and whistles.

I wanted to submit this pull request to adjust the formatting of the default error messages to better match [ActiveModel](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/locale/en.yml) & [ActiveRecord](https://github.com/rails/rails/blob/master/activemodel/lib/active_model/locale/en.yml) error messages.  If you look at the `en.errors.messages` path of both those locale files I linked to, you can see that none of the messages end with a period.  This is because sometimes those strings will be concatenated (via `to_sentence` or similar) if there are multiple errors in a field.  The current formatting used by simple_enum causes a period to appear in the middle of a sentence or comma-separate list of errors pertaining to a field.

I know this can be fixed by supplying a custom locale value in a local project, but I figured that with this being a gem based on ActiveModel, it should probably follow the same use cases of ActiveModel itself.  Thanks for the consideration!