Add the license to gemspec.

To see why this attribute is important in the gemspec, see the following
links:

* https://github.com/technicalpickles/capistrano-spec/issues/17
* http://www.benjaminfleischer.com/2013/07/12/make-the-world-a-better-place-put-a-license-in-your-gemspec/
Merge pull request #18 from jhbabon/add-license-to-gemspec

Add the license to gemspec.