fix test link

Author: tony

README.rst

@@ -186,7 +186,7 @@ API             http://libtmux.git-pull.com/api.html
 Changelog       http://libtmux.git-pull.com/history.html
 Issues          https://github.com/tony/libtmux/issues
 Travis          http://travis-ci.org/tony/libtmux
-Test Coverage   https://coveralls.io/r/tony/libtmux
+Test Coverage   https://codecov.io/gh/tony/libtmux
 pypi            https://pypi.python.org/pypi/libtmux
 Open Hub        https://www.openhub.net/p/libtmux-python
 License         `BSD`_.