README: little typo on instructions.

[javimbk]

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.