Fixed build-demo script.

Author: ppotaczek

package.json

@@ -16,7 +16,7 @@
   ],
   "scripts": {
     "test": "jest",
-    "build-demo": "browserify ./demo/demo.jsx -o ./demo/bundle.js -t [ babelify --presets [ env react ] ]",
+    "build-demo": "browserify ./demo/demo.jsx -o ./demo/bundle.js -t [ babelify --presets [ @babel/preset-env @babel/preset-react ] ]",
     "build": "webpack --config webpack.config.js",
     "release": "standard-version"
   },