Added Coveralls code coverage button to README and after_script command to travis.yml (snowplow#64)

Author: fblundun

.travis.yml

@@ -8,7 +8,10 @@ install:
   - "pip install httmock"
   - "pip install freezegun"
   - "pip install nose"
-  - "pip install coverage"
+  - "pip install coveralls"
   - "pip install -r requirements.txt"
 # command to run tests
-script: nosetests --with-coverage --cover-package=snowplow_tracker
+script:
+  nosetests --with-coverage --cover-package=snowplow_tracker
+after_success:
+  coveralls

README.rst

@@ -6,6 +6,9 @@ Python Analytics for Snowplow
     :target: https://travis-ci.org/snowplow/snowplow-python-tracker
 .. image:: https://badge.fury.io/py/snowplow-tracker.png
     :target: http://badge.fury.io/py/snowplow-tracker
+.. image:: https://coveralls.io/repos/snowplow/snowplow-python-tracker/badge.png
+    :target: https://coveralls.io/r/snowplow/snowplow-python-tracker
+
 
 Overview
 ########