use coveralls for test coverage reporting with Travis

.coveragerc

@@ -0,0 +1,8 @@
+# configuration for coverage.py
+[run]
+source = mozregression
+
+[report]
+exclude_lines =
+    # Don't complain if tests don't hit defensive assertion code
+    raise NotImplementedError

.travis.yml

@@ -4,6 +4,10 @@ python:
     - "2.7"
 
 install:
-    pip install mock
+    pip install mock coverage coveralls
 
-script: python setup.py test
+script:
+    coverage run setup.py test
+
+after_success:
+    coveralls

README.md

@@ -2,6 +2,7 @@ mozregression is an interactive regression range finder for Mozilla nightly and
 
 [![PyPI version](https://badge.fury.io/py/mozregression.svg)](http://badge.fury.io/py/mozregression)
 [![Build Status](https://travis-ci.org/mozilla/mozregression.svg?branch=master)](https://travis-ci.org/mozilla/mozregression)
+[![Coverage Status](https://img.shields.io/coveralls/mozilla/mozregression.svg)](https://coveralls.io/r/mozilla/mozregression)
 
 For more information see: