Remove the hard gem dependency on Curb

Curb sucks. There's no reason to introduce this C-extension dependency to a project unless the end-user really wants it. And if you really want it, it's easy. add `gem 'curb-fu` just before `fastly` in your Gemfile. 
https://github.com/fastly/fastly-ruby/blob/master/lib/fastly/client.rb#L12-L16 ?