one liner fix green_onion.rb:58:in `skin_picker': undefined method `color' for String

My project is using `ruby-2.1.2` and I was getting this error:
```
$ green_onion skin http://www.google.com
/Users/rcorreia/.rvm/gems/ruby-2.1.2/gems/green_onion-0.1.4/lib/green_onion.rb:58:in `skin_picker': undefined method `color' for "http://www.google.com":String (NoMethodError)
	from /Users/rcorreia/.rvm/gems/ruby-2.1.2/gems/green_onion-0.1.4/lib/green_onion.rb:53:in `skin_visual_and_percentage'
	from /Users/rcorreia/.rvm/gems/ruby-2.1.2/gems/green_onion-0.1.4/lib/green_onion/cli.rb:31:in `skin'
	from /Users/rcorreia/.rvm/gems/ruby-2.1.2/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
	from /Users/rcorreia/.rvm/gems/ruby-2.1.2/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
	from /Users/rcorreia/.rvm/gems/ruby-2.1.2/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
	from /Users/rcorreia/.rvm/gems/ruby-2.1.2/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
	from /Users/rcorreia/.rvm/gems/ruby-2.1.2/gems/green_onion-0.1.4/bin/green_onion:4:in `<top (required)>'
	from /Users/rcorreia/.rvm/gems/ruby-2.1.2/bin/green_onion:23:in `load'
	from /Users/rcorreia/.rvm/gems/ruby-2.1.2/bin/green_onion:23:in `<main>'
	from /Users/rcorreia/.rvm/gems/ruby-2.1.2/bin/ruby_executable_hooks:15:in `eval'
	from /Users/rcorreia/.rvm/gems/ruby-2.1.2/bin/ruby_executable_hooks:15:in `<main>'
```

The change I applied was suggested here: 
https://github.com/sickill/rainbow/blob/master/README.markdown#string-mixin