Add with_polymorphism and with_cascading_callbacks association matcher

Includes tests

Also update RSpec to >= 2.14 since it's been released now
Add more environments to Travis; add "allow_failures" on unstable environments
Add Association.cyclic and .stored_as matchers. (stored_as is a valid attribute on embedded relations, it is not deprecated.)
Merge pull request #110 from johnnyshields/master

Improved assocition matcher: with_polymorphism, with_cascading_callbacks, stored_as, cyclic