Add license to gemspec, License as MIT

The runtime dependent gems are licensed as New BSD and MIT.

This gem's license omits a line from the New BSD. I'm not an attorney so I'm not sure what to call it, hence 'modified'

Also see https://github.com/pivotal/LicenseFinder/blob/master/lib/data/licenses/NewBSD.txt and https://github.com/bf4/gemproject/issues/1