bump grunt-cdn to resolve some incorrect replacements

Was seeing some interesting replacements when using "cdnify": 

Changing /width=device-width -> https://storage.googleapis.com/mailer-cdn/width=device-width
Changing /text/html;%20charset=UTF-8 -> https://storage.googleapis.com/mailer-cdn/text/html;%20charset=UTF-8

I bumped the version of grunt-cdn to the latest (0.6.1) and those faulty replacements disappeared.