Fix stack level too deep error due to namespacing

If you have a state machine with a "failed" event and you transition it
and it fails to transition, AASM will call the "failed" method it
defines but will hit your transition instead and blow the stack.

All other methods in AASM are prefixed with AASM to avoid this, so this
patch does the same thing with the private methods `failed` and `fired`.

This bit me on an existing state machine I had when upgrading versions. I think its common enough to have a "failed" event that solution should be on the library and not the user.