Remove redundant generator tasks

- Remove redundant statement to mount KriKri engine
- Remove invocation of krikri:install:migrations

Regarding the migrations, it's probably still up for discussion,
but I've changed the migrations to live inside the KriKri gem,
per:
http://pivotallabs.com/leave-your-migrations-in-your-rails-engines/
I just need it to work one way or the other at the moment because
installations (and CI tests) are erroring out.