reset FactoryGirl::$createdClasses after flush

After a `FactoryGirl::flush()`, the `FactoryGirl::$createdClasses` should be reset to empty.

Say in the first test case a Foo object was created, and it was `flush`ed after the test case.
Then we created only a Baz object in the second test case. By the second `flush` it should only clear the Bar objects, since there was no Foo object created since the first `flush`