fixes failing require and postgres and sqlite3 test delete_all test

* this file composite_primary_keys/active_model/dirty does not exist
* Checking the line exception message is not really ideal as it will
depend on each drivers' implementation
Merge pull request #248 from StupidCodeFactory/fix_postgres_and_sqlite_delete_all_test

fixes failing require and postgres and sqlite3 test delete_all test
Switch from .rvmrc to .ruby-version (RVM recommends this)
add a test for `id_was`
Remove singleton classes on CPK relations against Rails 4.1

* Extract CPK methods to its own module and extending instead of opening the singleton class.

* Fix `Marshal.dump`. Makes caching work again.
Merge pull request #252 from nhocki/delegate_rails_41

Remove singleton classes on CPK relations against Rails 4.1
Merge pull request #250 from codeodor/master

Add a test for id_was and switch from .rvmrc to .ruby-version style
merge master and fix for ActiveRecord 4.2.0.beta4