Plugin not working at all

The plugin is not working at all for, it doesn't print anything. I guess this must be some kind of upstream change in pytest. However, the problem gets fixed when you execute the write inside a `capture_manager` context, i.e.:

``` python
with self._capture_manager.global_and_fixture_disabled():
    self._reporter.write_line(msg)
```

I can open a PR if you want. 