added the rubyracer gem to solve the javascript runtime problem

error without the rubyracer gem:

.rvm/gems/ruby-1.9.3-p392/gems/execjs-1.3.0/lib/execjs/runtimes.rb:50:in `autodetect': Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. (ExecJS::RuntimeUnavailable)