Ignore the ParallelEach changes when running Minitest4

Hi there!

Updating our app to rails 4.0 has messed up with our test-queue setup a bit. After the update there'd be some workers that would finish really quick and others that would take ages.

Digging a bit I found out that Rails 4 automatically requires minitest/parallel_each ([first this](https://github.com/rails/rails/blob/v4.0.10/activesupport/lib/active_support/test_case.rb#L9) and [then this](https://github.com/rails/rails/blob/v4.0.10/activesupport/lib/active_support/testing/isolation.rb#L2)). 

That changes `_run_suites` to [call partition first](https://github.com/seattlerb/minitest/blob/644a52fd0aa0205c8767829ca8afbd8c158f9ff2/lib/minitest/parallel_each.rb#L62), which exhausts the iterator. That means all suites are distributed at the beginning instead of being
pulled as workers are available.

I see the minitest5 runner does seem to handle all these parallel test_order tests much better, so I figure out this problem only affects rails 4.0 with minitest 4.

What do you think of this solution? It does the trick in our case.