Add license to gemspec, is MIT

RubyGems.org doesn't report a license for your gem.  This is because it is not specified in the [gemspec](https://github.com/rubygems/rubygems/pull/713) of your last release.

via e.g.

    spec.license = 'MIT'
    # or
    spec.licenses = ['MIT', 'GPL-2']

Including a license in your gemspec is an easy way for rubygems.org and other tools to check how your gem is licensed.  As you can imagine, scanning your repository for a LICENSE file or parsing the README, and then attempting to identify the license or licenses is much more difficult and more error prone. So, even for projects that already specify a license, including a license in your gemspec is a good practice. See, for example, how [rubygems.org uses the gemspec to  display the rails gem license](https://rubygems.org/gems/rails).

There is even a [License Finder gem](https://github.com/pivotal/LicenseFinder) to help companies/individuals ensure all gems they use meet their licensing needs. This tool depends on license information being available in the gemspec.  This is an important enough issue that *even Bundler now generates gems with a default 'MIT' license*.

I hope you'll consider specifying a license in your gemspec. If not, please just close the issue with a nice message. In either case, I'll follow up. Thanks for your time!

Appendix:

p.s. In case you're wondering how I found you and why I made this issue, it's 1) because I maintain metric_fu and 2) because I'm collecting stats on gems (I was originally looking for download data) and decided to collect license metadata,too, and [make issues for gemspecs not specifying a license as a public service :)](https://github.com/bf4/gemproject/issues/1). See the previous link or my [blog post about this project for more information](http://www.benjaminfleischer.com/2013/07/12/make-the-world-a-better-place-put-a-license-in-your-gemspec/).