Call str() on unicode strings

Hi David. When I tried to set this up on a Mac OS X 10.8.4, running Python 2.7.2, I was getting errors running $buildbot start master, specifically "ForceScheduler builderNames must be a list of strings: [u'Android_Master', u'Android_Master',......".  Adding str() here solved the issue, but I'm not sure if this is a problem with my environment only and should be merged or what.