Use Minitest::StatisticsReporter

Minitest::StatisticsReporter#report already gathers the statistics, so
we can delegate stat-gathering to it and just call our notifier with the
stats.
Merge pull request #65 from kejadlen/feature/minitest-reporter

Use `Minitest::StatisticsReporter` instead of `Minitest::Reporter` for Minitest > 5.0