Update license attribute

specifying the type and URL is deprecated:

https://docs.npmjs.com/files/package.json#license
http://npm1k.org/

package.json

@@ -18,6 +18,6 @@
     "btoa": "bin/btoa.js"
   },
   "main": "index",
-  "license": "Apache2",
+  "license": "Apache-2.0",
   "version": "1.1.2"
 }