Adds license attribute to the sample gemspec

Including a license in your gemspec is a good practice.
[Make the world a better place; put a license in your gemspec](http://www.benjaminfleischer.com/2013/07/12/make-the-world-a-better-place-put-a-license-in-your-gemspec/)
Merge pull request #54 from zjjz/feature-spec-add-license

Adds license attribute to the sample gemspec