Add license attribute
Add license attribute

specifying the type and URL is deprecated:

https://docs.npmjs.com/files/package.json#license
http://npm1k.org/
PTaylour/natural,mcanthony/natural,walkingdoctors/natural,kjhenner/natural,PMSI-AlignAlytics/natural,ahmedshuhel/natural,Hugo-ter-Doest/natural,dpraimeyuu/natural,NaturalNode/natural,levy5674/natural,polymaths/natural,karimsa/natural,moos/natural,itamayo/natural