jekyll: update instructions

Seems it got a bit easier on modern ubuntu.

Author: tfnico

_includes/README.md

@@ -15,8 +15,7 @@ [email protected] if appropriate).
 
 # Development
 
-* Make sure you've got ruby2 (packages are available, have to replace
-`/usr/bin/ruby` and `/usr/bin/gem` with their v2 options)
+* Make sure you've got ruby2 with dev-packages installed
 *`sudo gem install bundler`
 * Clone this repo
 *`sudo apt-get install zlib1g-dev` # ref [1]