Require at least version 0.18.0 of thor

simply gem installing tmuxinator, it will fail to run due to the `package_name` method being undefined:

```
/Users/wil/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/tmuxinator-0.6.3/lib/tmuxinator/cli.rb:12:in `<class:Cli>': undefined method `package_name' for Tmuxinator::Cli:Class (NoMethodError)
	from /Users/wil/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/tmuxinator-0.6.3/lib/tmuxinator/cli.rb:2:in `<module:Tmuxinator>'
	from /Users/wil/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/tmuxinator-0.6.3/lib/tmuxinator/cli.rb:1:in `<top (required)>'
	from /Users/wil/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /Users/wil/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /Users/wil/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/tmuxinator-0.6.3/lib/tmuxinator.rb:9:in `<top (required)>'
	from /Users/wil/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /Users/wil/.rbenv/versions/1.9.3-p194/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /Users/wil/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/tmuxinator-0.6.3/bin/mux:5:in `<top (required)>'
	from /Users/wil/.rbenv/versions/1.9.3-p194/bin/mux:23:in `load'
	from /Users/wil/.rbenv/versions/1.9.3-p194/bin/mux:23:in `<main>'
```
Merge pull request #133 from wil/patch-1

Require at least version 0.18.0 of thor.