Add minitest 4.x as an explicit dependency

In the CI builds (currently with CircleCI), we use minitest 4.7. Using
minitest 5 introduces an error, so we should lock to 4.x for the time
being.

See: https://github.com/seattlerb/minitest/issues/28
Merge pull request #293 from Nitrodist/use-minitest-4

Add minitest 4.x as an explicit dependency