tests: switch to Minitest 5

Update the test suite to use the new Minitest 5 framework API. Remove
the last references to Test::Unit and standardize on "Minitest"
everywhere.
travis: remove Rubinius 1.9

Travis CI has deprecated rbx-19mode. Replace it with an updated Rubinius
version.
tests: skip test failure on Rubinius
Merge pull request #3 from ktdreyer/minitest

tests: switch to Minitest 5 and Rubinius 2 in Travis