Point coverage badge link to master

README.rst

 .. image:: https://travis-ci.org/edx/edx-tim.png?branch=master
     :alt: Travis build status
 
-.. image:: https://coveralls.io/repos/edx/edx-tim/badge.png
-  :target: https://coveralls.io/r/edx/edx-tim
-  :alt: Coverage badge
+
+.. image:: https://coveralls.io/repos/edx/edx-tim/badge.png?branch=master
+    :target: https://coveralls.io/r/edx/edx-tim?branch=master
+    :alt: Coverage badge
+
 
 This is an initial prototype for redesigning Peer Grading and general Open Ended
 Submission Evaluation. This project is in the early stages of development and is