Fix Gemnasium's devDependencies badge

Currently, if a gem is showing 'devDependencies', for example: ![](https://gemnasium.com/dorentus/bnet-authenticator.svg) (devDependencies up-to-date), in shields it becomes: ![](http://img.shields.io/gemnasium/dorentus/bnet-authenticator.svg?style=flat) (dependency invalid).

This patch will fix this minor issue.