Fix exceptions.DeprecationWarning: Argument strings and environment keys...

When I start buildbot I have more than 100 log lines like this in my twistd.log: 
```
 2014-12-09 16:39:23+0100 [-] /home/xavierd/Projects/myGitHub/buildbot-work/sandbox/local/lib/python2.7/site-packages/twisted/internet/utils.py:30: exceptions.DeprecationWarning: Argument strings and environment keys/values passed to reactor.spawnProcess should be str, not unicode.
```