pivotaltracker: Update areAllReviewsApproved

Added a check to make sure the story is labeled as implemented to ever return
true from this function. Unless the label is there, the developer is allowed to
add more review requests in the future, according to the workflow, so the story
cannot be transitioned yet.

Change-Id: ccd1e6e13b
pivotaltracker: Update transitionToNextState

The function now only adds the 'reviewed' label now, it also removes the
'implemented' label when that one is present.

Change-Id: e918853ff1