Extracted child process management out of Resque::Worker into
Resque::ChildProcess.

This is an attempt to reduce the complexity of Resque::Worker.

* Extracted logic for a forked child process into a dedicated class
* Made Worker#will_fork? public due to legacy test stubbing
* Moved the majority of the logic from Worker#fork_for_child into ChildProcess#fork_and_perform
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
Merge pull request #998 from einarj/childcare

Childcare