re-add marked as a dev dependency

Error when running Grunt before change: 

Running "ensure-installed" task
bower downloading http://sinonjs.org/releases/sinon-1.7.3.js
...
bower checking out text#2.0.10
Fatal error: Invalid Version: 6d6c5e08a1295fee047849abbad927c7f0de99bd

Fixed with this change.