Remove Goal.complete_for

This class method has some serious implications in that it overrides the
default scope of Gamification::Goal, causing it to be scoped to its
return value in subsequent invocations.

For example:

    Gamification::Goal.count # => 3
    Gamification::Goal.complete_for user
    Gamification::Goal.count # => 1
Merge pull request #5 from jgorset/fix/remove-goal.complete_for

Remove Goal.complete_for