Add Followup and change gemspec

On http://robots.thoughtbot.com/rubys-pessimistic-operator they say 

> `gem 'thin',  '~> 1.1'`
> ~> 1.1 means that when you bundle install, you’ll get the highest-released gem version of thin between the range >= 1.1 and < 2.0.

This is what I was trying to go for with the previous `'~> 0', ">= 0.20"`, anything in the 0 major at least 0.20, so I think `~> 0.20` is the proper way to say that. Yeah?