Update test coverage link from coveralls to codecov. Fix #4143

Uses the same link as the badge at the top of the README.

README.rst

@@ -32,7 +32,7 @@ Features
 * Workflow support
 * An extensible `form builder <http://docs.wagtail.io/en/latest/reference/contrib/forms/index.html>`_
 * Multi-site and multi-language support
-* Excellent `test coverage <https://coveralls.io/r/torchbox/wagtail?branch=master>`_
+* Excellent `test coverage <http://codecov.io/github/wagtail/wagtail?branch=master>`_
 
 Find out more at `wagtail.io <http://wagtail.io/>`_.