I’ve seen a few questions about this error lately. It’s pretty self-explanatory, but here are the details in the most straight-forward manner I could manage.
bq. https://github.com/sstephenson/execjs for a list of available runtimes.
Visiting http://github.com/sstephenson/execjs will give you the answer. But for the lazy, just add this to your Gemfile
gem ‘therubyracer’, :require => ‘v8’