update to current jekyll and gems, remove octopress

[haslinger]

I wanted to read your tutorial, but Github pages' switch to https made it basically unreadable.
So I thought, why not fix that detail. I learned, that it still used octopress, which I kicked out,
updated Jekyll, kicked out any pygments or kramdown options, as highlighting and markdown parsing are typically OK in current Jekyll. And gave it a try.
It seems quite fine for me (though I don't know the original, unbroken version), at least readable again and surprisingly enough even the sitemap worked. So I thought, why not create a PR?

Thanks for your work on the tutorial. Looking forward to read it right now!

[brunoruviaro]

Hi, sorry I never replied to your post. The tutorial is still accessible at http://brunoruviaro.github.io/ardour4-tutorial/tags/ -- I haven't updated it since. I probably need to do a major upgrade to everything including content, since Ardour is already at version 5.