Easy dev install

@jeroenvanbaarsen  just tried the following:

1. Removing therubyracer from gemfile
2. Install node (brew install node or sudo apt-get install nodejs)
3. Run $ spinach features/

All worked as before, so this would mean people have to install the node dependency on their local machine, but in my experience this is much easier than installing libv8, especially for non ruby-devs. /cc @jacobvosmaer

See merge request !262