Recently I stumbled over a tool Coveralls. It helps monitoring your test coverage. You can integrate it e.g. with your Travis-CI builds by using a plugin for your favorite build tool. Needless to say that they also provide Travis-CI similar badge to show off your current test coverage near your current build status in your README file at GitHub.
Their docs sadly aren’t very focused on Java tools like Maven or Gradle, so you have to find a working plugin on your own (or code a new one by using the Coveralls API). I tried the coveralls-gradle-plugin for my gradle-debian-plugin and it works like a charm! Just have a look at the list of results - showing some potential to improve test coverage ;-)