Add missing license to gemspec, is MIT

See 

- https://github.com/bf4/gemproject/issues/1
- http://guides.rubygems.org/specification-reference/#license=
- http://www.benjaminfleischer.com/2013/07/12/make-the-world-a-better-place-put-a-license-in-your-gemspec/
Merge pull request #55 from bf4/add_license_to_gemspe

Add missing license to gemspec, is MIT