Fix mixed content rendering issues

If accessing the gh-pages url with an HTTPS connection, stylesheets and
scripts will not load due to mixed content issues [1].

A solution to this is to have a protocol agnostic URL [2]. I’ve updated
the production_url variables to be protocol agnostic URLs.

[1]:https://support.mozilla.org/en-US/kb/how-does-content-isnt-secure-af
fect-my-safety
[2]:http://www.paulirish.com/2010/the-protocol-relative-url/