Fix broken specs on rails 4.1.0 on association matcher

Seems rails 4.1.0 treats has_and_belongs_to_many as a has_many :through
More info on:

https://github.com/rails/rails/issues/12582
http://edgeapi.rubyonrails.org/classes/ActiveRecord/Reflection/ClassMethods.html#method-i-reflect_on_association