Add npm files filter

package.json

@@ -30,6 +30,10 @@
   ],
   "license": "MIT",
   "main": "dist/js/select2.js",
+  "files": [
+    "src",
+    "dist"
+  ],
   "version": "4.0.0",
   "jspm": {
     "main": "js/select2",