Use unittest.main to run tests for correct exit code

When running the TestRunner instance directly, the script would report
failures to std{out,err} but exit with a success status despite test
failures, c.f.
https://travis-ci.org/tBaxter/vobject/jobs/44978065#L859-L859:

```
FAILED (failures=26)
The command "python test_vobject.py additional_tests" exited with 0.
```

Using `unittest.main`, things Should Just Work