Update license attribute

specifying the type and URL is deprecated:

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

Updated to "Apache-2.0" per https://spdx.org/licenses/

package.json

@@ -78,7 +78,7 @@
     "through2-map": "~1.4.0",
     "xmlbuilder": "~0.4.3"
   },
-  "license": "Apache 2.0",
+  "license": "Apache-2.0",
   "dependencies": {
     "bluebird": "^2.9.14",
     "chalk": "^1.0.0",
@@ -98,4 +98,4 @@
     "node": ">=0.8 <=0.12",
     "iojs": ">=1.5"
   }
-}
+}