Integration check

Implements tasks jumpup:integration:check, jumpup:integration:lock, jumpup:integration:unlock to block a user from integrating while another one is already integrating at same time. It is useful when the project have a lot of tests because it avoids the second user to receive the reject message from git, and have to run all tests again.

I did not put it on jumpup:start and jumpup:finish for compatibility reason. If everyone agrees, we can remove this verification from jumpup-heroku and then use this one as default.

@thiagogabriel @maurogeorge