Pin minitest to 4.7.0

We're not compatible with 5.
Remove hoptoad failure backend and removes configure for airbrake.

Hoptoad no longer exists, so we don't need it.

Now that we only have one backend, we pull the thoughtbot one up into
airbrake.

configure is gone because it just did global config. Instead of running
the block, do this instead:

    Resque::Failure.backend = Resque::Failure::Airbrake