Fix licenses in gemspec to Rubygems standard
Fix licenses in gemspec to Rubygems standard

According to [Rubygems](http://guides.rubygems.org/specification-reference/#license=), you should specify your license using an [SPDX ID](https://spdx.org/licenses/). This permits tools like [LicenseFinder](https://github.com/pivotal/LicenseFinder) to properly find the relevant data.

The only thing that this PR does is specify the existing MIT license in this way.
Phrogz/NeatJSON,Phrogz/NeatJSON,Phrogz/NeatJSON