Refactor model specs

PT https://www.pivotaltracker.com/story/show/76108962

- refactor model specs to uniform format, use shoulda-matchers
- delete empty model spec files
When using `is_expected.to_not allow_value`, you should separately test one value at a time. The line above will succeed when the first value (7) invalidates the object and the other two values will not be tested.

http://thoughtbot.github.io/shoulda-matchers/v2.7.0/Shoulda/Matchers/ActiveModel.html#allow_value-instance_method