run coveralls only after the py27 build which is the only environment which generates coverage data....not sure how coveralls is going to find that data....but lets see if it works.
use the twisted repo for the coveralls badge
Use full bash variable notation
Merge pull request #23 from twisted/coveralls-15

Submit coverage data to coveralls and add a badge (fixes #15)

Author: @wallrj
Reviewer: @exarkun