schemaless url for site.url variable

Github uses `https` to serve the website and content
loaded over `http` gets blocked. Since all assets
are using the `{{site.url}}` property to build their
locations, they are currently attempting to load over
`http` and are being blocked. This will fix that. 

The only thing I'm not sure about this breaking is `sitemap.xml`.
Not sure how to check without deploying.