Debug message raises TypeError when classname is None.

```
Traceback (most recent call last):
  File "~\Sublime Text 3\sublime_plugin.py", line 442, in is_enabled_
    ret = self.is_enabled()
  File "~\Sublime Text 3\Data\Packages\sublime-phpunit\phpunit.py", line 939, in is_enabled
    filename = self.find_test_file()
  File "~\Sublime Text 3\Data\Packages\sublime-phpunit\phpunit.py", line 588, in find_test_file
    Msgs.debug_msg("classname is: " + classname)
TypeError: Can't convert 'NoneType' object to str implicitly
```