Fix coveralls link

The link to coveralls was wrong and did not show the master branch.

README.md

@@ -1,5 +1,5 @@
 [![Stories in Ready](https://badge.waffle.io/mesosphere/marathon.png?label=ready,gui&title=Ready)](https://waffle.io/mesosphere/marathon?label=gui)
-# Marathon UI [![Build Status](https://travis-ci.org/mesosphere/marathon-ui.png?branch=master)](https://travis-ci.org/mesosphere/marathon-ui) [![Teamcity Snapshot UI Webjar Build Status](https://teamcity.mesosphere.io/app/rest/builds/buildType:%28id:Oss_Marathon_SnapshotUiWebjar%29/statusIcon)](https://teamcity.mesosphere.io/viewType.html?buildTypeId=Oss_Marathon_SnapshotUiWebjar&guest=1) [![Coverage Status](https://coveralls.io/repos/mesosphere/marathon-ui/badge.svg?branch=master&service=github)](https://coveralls.io/github/mesosphere/marathon-ui?branch=feature%2Fadd-code-coverage-task)
+# Marathon UI [![Build Status](https://travis-ci.org/mesosphere/marathon-ui.png?branch=master)](https://travis-ci.org/mesosphere/marathon-ui) [![Teamcity Snapshot UI Webjar Build Status](https://teamcity.mesosphere.io/app/rest/builds/buildType:%28id:Oss_Marathon_SnapshotUiWebjar%29/statusIcon)](https://teamcity.mesosphere.io/viewType.html?buildTypeId=Oss_Marathon_SnapshotUiWebjar&guest=1) [![Coverage Status](https://coveralls.io/repos/mesosphere/marathon-ui/badge.svg?branch=master&service=github)](https://coveralls.io/github/mesosphere/marathon-ui?branch=master)
 
 ![Marathon UI](https://raw.githubusercontent.com/mesosphere/marathon-ui/master/marathon-ui.png "Marathon UI")