Master minitest5

These are changes necessary to migrate the testsuite to Minitest 5.x (related to #865 and #867).

The only remaining caveat is Test::Unit::TestSuite in test/contest.rb. I am not sure what was it good for, but the test suite is passing without it, so I left it commented out. May be it should be removed entirely?
I think we have to change this and require minitest on 1.8 as well, does minitest 5 run on ruby 1.8?