[rnpm] Fix sourceDir for Android (react-native-maps#1313)

Overlooked this change closing out a few PR's last week.

Follow up for react-native-maps#1200

package.json

@@ -53,7 +53,7 @@
   },
   "rnpm": {
     "android": {
-      "sourceDir": "./android"
+      "sourceDir": "./lib/android"
     }
   }
 }