README: little typo on the instructions

I was checking out your gem today (great work btw, I'm currently working with it) and came across a little problem that may scare some newbies away to think that it does not work.

`spec/spec_helper.rb:60:in `<top (required)>': undefined method `ruby' for main:Object (NoMethodError)`

It was missing `""` in the `spec_helper.rb` so I thought I would fix it and just do a quick PR to save you the trouble.

Thanks again for your work.