Add files whitelist to babelrc is not in npm package. Fixes preactjs/preact-compat#294

Author: developit

package.json

@@ -8,6 +8,11 @@
     "build": "babel -f $npm_package_main -m umd --module-id $npm_package_amdName -s -o $npm_package_main < src/index.js",
     "release": "npm run -s build && git commit -am $npm_package_version && git tag $npm_package_version && git push && git push --tags && npm publish"
   },
+  "files": [
+    "src",
+    "index.js",
+    "index.js.map"
+  ],
   "repository": {
     "type": "git",
     "url": "https://github.com/developit/proptypes.git"