Add license information to gemspec

This increases the discoverability of what license this gem uses. See rubygems/rubygems.org#363 or [this blog post](http://www.benjaminfleischer.com/2013/07/12/make-the-world-a-better-place-put-a-license-in-your-gemspec/) for more info.
Merge pull request #289 from davidcelis/patch-1

Add license information to gemspec