Ruby on Rails 3.1 needs server-side ECMAScript because some gems of Rails 3.1 need it - CoffeeScript and sass. So you should install node.js. Especially, I recommend it for windows user because node.js provides windows binary.
You could uninstall abstract gem
Ruby on Rails 3.1 needs erubis 2.7.0, And this version of erubis is noe depending on abstract gem despite erubis 2.6.x depends it. So you could uninstall abstract gem if you upgrade Rails from 3.0.x.