installation docs: specify Python 2.7 as dep for Jekyll 2
installation docs: specify Python 2.7 as dep for Jekyll 2

Fixes #4036.

stomar/jekyll,derekgottlieb/jekyll,derekgottlieb/jekyll,jeffkole/jekyll,jeffkole/jekyll,jekyll/jekyll,zsyed91/jekyll,rlugojr/jekyll,zsyed91/jekyll,Strangehill/jekyll,bsmr-ruby/jekyll,zsyed91/jekyll,mrb/jekyll,noikiy/jekyll,floydpraveen/jekyll,rasa2011/rasa2011.github.io,stomar/jekyll,mrb/jekyll,tillgrallert/jekyll,jaybe-jekyll/jekyll,tillgrallert/jekyll,alex-kovac/jekyll,gencer/jekyll,Strangehill/jekyll,Kinghack/kinghack.github.com,floydpraveen/jekyll,brint/jekyll,jekyll/jekyll,x-way/jekyll,tomjohnson1492/jekyll,tomjohnson1492/jekyll,princeofdarkness76/jekyll,xcatliu/jekyllcn,rlugojr/jekyll,pmarsceill/jekyll,alfredxing/jekyll,ryanshaw/jekyll,princeofdarkness76/jekyll,tomjohnson1492/jekyll,darwin/jekyll,yihangho/jekyll,nasht00/jekyll,rasa2011/rasa2011.github.io,larryfox/jekyll,gencer/jekyll,jaybe-jekyll/jekyll,ajhit406/jekyll,pmarsceill/jekyll,noikiy/jekyll,LeuisKen/jekyllcn,darwin/jekyll,brint/jekyll,tillgrallert/jekyll,ducktyper/jekyll,chrisfinazzo/jekyll,floydpraveen/jekyll,noikiy/jekyll,brint/jekyll,Strangehill/jekyll,ajhit406/jekyll,ducktyper/jekyll,getandpost/jekyll.github.io,nasht00/jekyll,jeffkole/jekyll,xcatliu/jekyllcn,mrb/jekyll,x-way/jekyll,fulldecent/jekyll,chrisfinazzo/jekyll,getandpost/jekyll.github.io,jekyll/jekyll,jaybe-jekyll/jekyll,ajhit406/jekyll,codeclimate-testing/jekyll,alfredxing/jekyll,alex-kovac/jekyll,noikiy/jekyll,xiebinhqy/jklly_china,tillgrallert/jekyll,gencer/jekyll,alfredxing/jekyll,pmarsceill/jekyll,stomar/jekyll,tomjohnson1492/jekyll,Kinghack/kinghack.github.com,aarvay/jekyll,ajhit406/jekyll,rlugojr/jekyll,floydpraveen/jekyll,Kinghack/kinghack.github.com,chrisfinazzo/jekyll,xiebinhqy/jklly_china,mrb/jekyll,ZDroid/jekyll,rebornix/jekyll,pmarsceill/jekyll,yihangho/jekyll,fulldecent/jekyll,darwin/jekyll,tillgrallert/jekyll,nasht00/jekyll,derekgottlieb/jekyll,rlugojr/jekyll,ryanshaw/jekyll,xcatliu/jekyllcn,xiebinhqy/jklly_china,x-way/jekyll,zsyed91/jekyll,rebornix/jekyll,larryfox/jekyll,jekyll/jekyll,darwin/jekyll,nasht00/jekyll,bsmr-ruby/jekyll,LeuisKen/jekyllcn,xiebinhqy/jklly_china,x-way/jekyll,tomjohnson1492/jekyll,gencer/jekyll,alex-kovac/jekyll,ZDroid/jekyll,codeclimate-testing/jekyll,codeclimate-testing/jekyll,gencer/jekyll,chrisfinazzo/jekyll,aarvay/jekyll,brint/jekyll,rasa2011/rasa2011.github.io,alex-kovac/jekyll,bsmr-ruby/jekyll,fulldecent/jekyll,LeuisKen/jekyllcn,fulldecent/jekyll,ryanshaw/jekyll,stomar/jekyll,alex-kovac/jekyll,alfredxing/jekyll,princeofdarkness76/jekyll,Strangehill/jekyll,Strangehill/jekyll,jaybe-jekyll/jekyll,ryanshaw/jekyll,rebornix/jekyll,aarvay/jekyll,larryfox/jekyll,ducktyper/jekyll,yhironaka/yhironaka.github.io,getandpost/jekyll.github.io,bsmr-ruby/jekyll,xcatliu/jekyllcn,jeffkole/jekyll,princeofdarkness76/jekyll,derekgottlieb/jekyll,ZDroid/jekyll,yihangho/jekyll