teardown must return :ok or {:ok, keywords}

Otherwise, the test will fail with a runtime error:

```
  1) test the truth (CallbacksTest)
     ** (RuntimeError) expected ExUnit callback in CallbacksTest to
return :ok  or { :ok, keywords }, got true instead
```
Merge pull request #150 from KeeperPat/bugfix/callbacks-test-with-meta-information

teardown must return :ok or {:ok, keywords}