Feature: Implement locking an environment to a ref

Heya

I've added a feature that allows a user to configure locking a git ref (our use case: a git tag) to an environment. I've added some extra test coverage and cleaned up some unused constructor args while I was digging in the area.

rspec tests all pass, although I did have some trouble running `rake spec:system` (although the failures are in a different area).

Cheers!