Cache bust assets

[philnash]

I noticed that @matiasgarciaisaia had referenced my blog post in #12 so I thought I'd take a crack at the task. This PR does a number of things to improve front end performance and avoid caching issues with CSS and JS.

• Loads fonts using a tag instead of @import in the CSS
• Stops loading Cousine weight 400, it isn't used
• Tidies up the pygments colour files using some Sass nesting
• Generates a single, minified stylesheet file with a digest in the filename
• Generates a single, minified javascript file with a digest in the filename

Ensure that when you build the site, JEKYLL_ENV is set to production otherwise there will be no minifying and no digest.

[philnash]

Hmm... this failed because Rack 2.0.1 requires Ruby >= 2.2.2 and the build is being run with 2.2.0.

I see that Travis is doing the build and then deploying (when on the master branch), I'll update the Ruby version to the latest in the 2.2.x series and try again.

[vtiffenberg]

The redesign has diverged quite a bit from master, so instead of merging this pull request I applied its suggestions in 9fd9cc, a00cd9 and 613b23, and will close.
Thanks @philnash for the contribution.

[philnash]

Thanks @vtiffenberg!

[matiasgarciaisaia]

(I really should have taken at least 20 seconds to thank you, @philnash, instead of waiting to work on this. Anyways - thank you very much! 🎉 )

[philnash]

No worries! Just glad to help out. Let me know if there's anywhere else I can pitch in.