Add source rubygems to the Gemfile
Make sure that the reverse denormalization doesn't blow up

Edge case bug...? Here's what happened:

Suppose you have a generic class User and subclasses for roles
Manager, Employee.

Suppose that Manager and Employee each belong_to Employer, but
that User and other subclasses do not.

Now consider a new class Store which belongs_to User (or, more
specifically it either belongs_to Manager or belongs_to Employee).

We'd like to use the denormalization plugin inside Store...
  denormalize_association :employer, :from => :user

However, when the reverse denormalization call executes on a
non-Manager, non-Employee User object, the code dies as this
object does not respond to :employer or to :employer_id_changed?

This commit prevents this problem.
Bump version