Update README

[skip ci]

Author: thomas-moulard

README.md

@@ -2,7 +2,7 @@ dynamic-graph
 =============
 
 [![Build Status](https://travis-ci.org/stack-of-tasks/dynamic-graph.png)](https://travis-ci.org/stack-of-tasks/dynamic-graph)
-[![Coverage Status](https://coveralls.io/repos/stack-of-tasks/dynamic-graph/badge.png)](https://coveralls.io/r/stack-of-tasks/dynamic-graph)
+[![Coverage Status](https://coveralls.io/repos/stack-of-tasks/dynamic-graph/badge.png?branch=master)](https://coveralls.io/r/stack-of-tasks/dynamic-graph?branch=master)
 
 
 This software provides an efficient way to modelize a C++ data-flow.