fix removing entire twisted when uninstalling cyclone

This is [issue86](https://github.com/fiorix/cyclone/issues/86) related.

I found a solution in Ralph Meijer's Wokkel repo - look at this [commit](https://github.com/ralphm/wokkel/commit/bb83eb66896e99848d4426f6dc8b23b88f0b35bc#setup.py)

It seems to work as expected now. In new virtualenv:

```bash
$ pip install twisted
....
$ pip install git+https://github.com/FZambia/cyclone.git@plugin_fix
...
$ pip uninstall cyclone
Uninstalling cyclone:
  /private/var/www/different/python/cyclone/issue86/env/bin/cyclone
  /private/var/www/different/python/cyclone/issue86/env/lib/python2.7/site-packages/cyclone
  /private/var/www/different/python/cyclone/issue86/env/lib/python2.7/site-packages/cyclone-git_2013011801-py2.7.egg-info
  /private/var/www/different/python/cyclone/issue86/env/lib/python2.7/site-packages/twisted/plugins/cyclone_plugin.py
  /private/var/www/different/python/cyclone/issue86/env/lib/python2.7/site-packages/twisted/plugins/cyclone_plugin.pyc
Proceed (y/n)? 
...
```