Edit package.json license to match SPDX id

See [NPM package.json spec for licenses](https://docs.npmjs.com/files/package.json#license) and [SPDX license IDs](https://spdx.org/licenses/)

X-ref: apache/cordova-plugin-device#48

 This closes #24
diff --git a/package.json b/package.json
index 481daa9..5d2ad8a 100644
--- a/package.json
+++ b/package.json
@@ -28,5 +28,5 @@
     }
   },
   "author": "Apache Software Foundation",
-  "license": "Apache 2.0"
+  "license": "Apache-2.0"
 }