Make tests run on Ruby 2.2.x

1. Added selective require of test-unit gem vs. minitest
2. Added closing `true` statement to Mocha `expects(:foo).with {...}` expressions, because `assert_equal` doesn't return a truthy value
3. Don't require "turn" on Ruby 2.2
4. Don't run integration tests against ActiveModel/Record 3.0 on Ruby 2.2
5. Don't run the Rails instrumentation test on Ruby 2.2 (should be investigated)