Use https mailchimp styling

This is a small tweak to use the https version of mailchimp's standard stylesheet to get around security issues in production.

![screenshot 2014-12-29 18 01 07](https://cloud.githubusercontent.com/assets/2766324/5574071/bb4a39b2-8f84-11e4-9538-97e91da75301.png)

Resolves #272!