Require test/unit/assertions instead of test/unit.

Fixes problem analogous to the one described in
https://github.com/thoughtbot/shoulda/issues/223
Merge branch 'master' of https://github.com/kammerer/flexmock into kammerer

* 'master' of https://github.com/kammerer/flexmock:
  Require test/unit/assertions instead of test/unit.