quick fix: ubuntu lib for rails 3.2.7 updated fresh install

if you are using webkit-driver for selenium in your rails project under ubuntu 12.04 lts
you may need follow lib

sudo aptitude install libqt4-dev libqtwebkit-dev

引用

Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable)

    gem 'therubyracer', '~> 0.9.9'

EDIT: I have also tried adding to the file with:

    gem 'therubyracer', :platforms => :ruby

sudo apt-get install node.js