Specify license on gemspec

[bbonamin]

Hey there,

We're using an automated tool called License Finder (https://github.com/pivotal/LicenseFinder) and your gem comes up as "Unknown". By adding the license to the gemspec file you would help immensely users of this and other automated tools to figure out whether a gem can be used inside another project.

Thanks!

[jeremyf]

ping @foxycoder

[wearethefoos]

Thanks, I'll roll an update with this!