Split the gem in 3

[casperisfine]

Because both minitest and rspec integration are provided by the same gem, we can't define explicit dependencies in the gemspec, even though that would make testing much much easier.

I think we should split in 3 gems:

• ci-queue: Keep all the queue implementations, depends on pretty much nothing.
• minitest-queue: depends on ci-queue, a specific minitest and minitest-reporters.
• rspec-queue: depends on ci-queue and a specific rspec version.