Ignore the ParallelEach changes when running Minitest4

Rails 4 automatically requires minitest/parallel_each and that changes
_run_suites to call partition first, which exhausts the iterator. That
means all suites are distributed at the beginning instead of being
pulled as workers are available.
Merge pull request #14 from mrsimo/minitest-4-parallel-each-messes-up

Ignore the ParallelEach changes when running Minitest4