Use Rails 4.2-compatible method for primary key

While testing against the Rails 4.2 beta (beta4), specs were failing due
to the use of the `#primary` method against database columns to
determine the primary key. Calling `.primary_key` on the record's class
successfully fetches the primary key column name in Rails 3 and 4 in a
way that is compatible with Rails 4.2.
Merge pull request #19 from geoffharcourt/rails-4-2

Use Rails 4.2-compatible method for primary key