Make SIGINTs nicer.

Previously, a SIGINT would cause a stack trace:

    [$]> ../license-generator/bin/licgen wtfpl-2.0
    Authors: ^C/Users/pearson/.rvm/gems/ruby-1.9.2-p180/gems/thor-0.14.6/lib/thor/shell/basic.rb:42:in `gets': Interrupt
            from /Users/pearson/.rvm/gems/ruby-1.9.2-p180/gems/thor-0.14.6/lib/thor/shell/basic.rb:42:in `ask'
            from /Users/pearson/.rvm/gems/ruby-1.9.2-p180/gems/thor-0.14.6/lib/thor/shell.rb:69:in `ask'
            from /Users/pearson/Documents/license-generator/lib/license-generator/app.rb:46:in `option'
            from /Users/pearson/Documents/license-generator/lib/license-generator/app.rb:21:in `method_missing'
            from /Users/pearson/.rvm/gems/ruby-1.9.2-p180/gems/thor-0.14.6/lib/thor/task.rb:22:in `run'
            from /Users/pearson/.rvm/gems/ruby-1.9.2-p180/gems/thor-0.14.6/lib/thor/task.rb:108:in `run'
            from /Users/pearson/.rvm/gems/ruby-1.9.2-p180/gems/thor-0.14.6/lib/thor/invocation.rb:118:in `invoke_task'
            from /Users/pearson/.rvm/gems/ruby-1.9.2-p180/gems/thor-0.14.6/lib/thor.rb:263:in `dispatch'
            from /Users/pearson/.rvm/gems/ruby-1.9.2-p180/gems/thor-0.14.6/lib/thor/base.rb:389:in `start'
            from ../license-generator/bin/licgen:7:in `<main>'

    [$]>

This is confusing, because pressing ctrl-c is a perfectly normal thing to do,
and a stack trace indicates that something went terribly wrong (even if it
didn't).  Worse, it's ugly!

Now, thanks to [the smallest gem in existence][0], this is no longer the case:

    [$]> ../license-generator/bin/licgen wtfpl-2.0
    Authors: ^C
    [$]>

[0]: https://github.com/xiongchiamiov/nice-sigint