Update installation.md
Update installation.md

1. Addition of *Running Jekyll on Ubuntu* section, to address Ubuntu stumbling block as per https://github.com/jekyll/jekyll/issues/5719.
2. Restructuring, and I hope I understood correctly when NodeJS/Python are/aren't required.
3. Gentler wording – it's probably not a good idea to tell punters who hit this page because they ran into trouble that installing Jekyll *is* easy and straight-forward; it *ought to be* straight-forward. (There's always the potential for pain and confusion if not all dependencies are in place.)
getandpost/jekyll.github.io,gencer/jekyll,alex-kovac/jekyll,Strangehill/jekyll,tomjohnson1492/jekyll,chrisfinazzo/jekyll,rlugojr/jekyll,ryanshaw/jekyll,chrisfinazzo/jekyll,tomjohnson1492/jekyll,chrisfinazzo/jekyll,x-way/jekyll,gencer/jekyll,jaybe-jekyll/jekyll,x-way/jekyll,xiebinhqy/jklly_china,Strangehill/jekyll,fulldecent/jekyll,rlugojr/jekyll,tomjohnson1492/jekyll,jekyll/jekyll,tillgrallert/jekyll,tillgrallert/jekyll,alex-kovac/jekyll,alex-kovac/jekyll,jekyll/jekyll,ryanshaw/jekyll,gencer/jekyll,xiebinhqy/jklly_china,rasa2011/rasa2011.github.io,rlugojr/jekyll,fulldecent/jekyll,fulldecent/jekyll,Strangehill/jekyll,gencer/jekyll,tomjohnson1492/jekyll,alex-kovac/jekyll,tillgrallert/jekyll,jaybe-jekyll/jekyll,jekyll/jekyll,chrisfinazzo/jekyll,Strangehill/jekyll,xiebinhqy/jklly_china,Strangehill/jekyll,tomjohnson1492/jekyll,jekyll/jekyll,jaybe-jekyll/jekyll,ryanshaw/jekyll,tillgrallert/jekyll,gencer/jekyll,x-way/jekyll,alex-kovac/jekyll,jaybe-jekyll/jekyll,getandpost/jekyll.github.io,getandpost/jekyll.github.io,x-way/jekyll,rlugojr/jekyll,yhironaka/yhironaka.github.io,tillgrallert/jekyll,xiebinhqy/jklly_china,rasa2011/rasa2011.github.io,fulldecent/jekyll,ryanshaw/jekyll,rasa2011/rasa2011.github.io