Add Gemnasium badge to readme

Gemnasium syncs with your GitHub repositories. It parses your projects'
dependencies and notifies you when new versions are released.

Change-Id: I008cdc1b77b8eb1a461d378f69ccced12fa0bf98
Reviewed-on: https://gerrit.causes.com/33805
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>

Author: lencioni

README.md

@@ -1,6 +1,7 @@
 [![Gem Version](https://badge.fury.io/rb/overcommit.png)](https://badge.fury.io/rb/overcommit)
 [![Build Status](https://travis-ci.org/causes/overcommit.png)](https://travis-ci.org/causes/overcommit)
 [![Code Climate](https://codeclimate.com/github/causes/overcommit.png)](https://codeclimate.com/github/causes/overcommit)
+[![Dependency Status](https://gemnasium.com/causes/overcommit.png)](https://gemnasium.com/causes/overcommit)
 
 # Overcommit