Load Google font with protocol relative URL

This allows for HTTPS serving of Shout.

With the specified 'http' protocol you would get the following error when loaded over HTTPS: 
```
[blocked] The page at 'URL-HERE' was loaded over HTTPS, but ran insecure content from 'http://fonts.googleapis.com/css?family=Lato:400,700': this content should also be loaded over HTTPS.
```

See this post by @paulirish about [the protocol relative url](http://www.paulirish.com/2010/the-protocol-relative-url/).