Add license information to gemspecs

I added the license information to the gemspecs.

I picked MIT for therubyrhino because of the [MIT license in the README](https://github.com/pivotal-sharknado/therubyrhino/blob/cac2b9512a5d82579b5cdb7f1c677a0ae6b0d590/README.md#license)

I picked MPL-2.0 for therubyrhino_jar because of the license information in [/jar/LICENSE.txt](https://github.com/pivotal-sharknado/therubyrhino/blob/cac2b9512a5d82579b5cdb7f1c677a0ae6b0d590/jar/LICENSE.txt)