CHORE: updated Rails dev dependency to 4.0.3
CHORE: Update tests to be RSpec 3 compliant

If the active_model_form_objects gem was used in a project that was
using RSpec 3 then the included `form_object` shared_examples would
break and show a number of deprecation notices. These tests are now
fully compliant with RSpec 3.

Changes included:
* Replace `should` assertions with `expect` syntax
* Remove `its` style statements
* Replace `should` style message expectations with `expect` syntax