Add modern license to package.json.
Add modern license to package.json.

See https://docs.npmjs.com/files/package.json#license

Without this change newer versions of `npm` write out a warning about the
license missing.

adamstortz/swagger-js-codegen,wcandillon/swagger-js-codegen,wcandillon/swagger-js-codegen,adamstortz/swagger-js-codegen