Fixes Webpack UMD using undefined global object

Applies webpack/webpack#6784 (comment)

See webpack/webpack#8625 for cause

Author: olofsvt

webpack.config.prod.js

@@ -13,6 +13,7 @@ module.exports = {
     publicPath: 'dist/',
     library: 'ReactSimpleChatbot',
     libraryTarget: 'umd',
+    globalObject: 'typeof self !== \'undefined\' ? self : this',
   },
   resolve: {
     extensions: ['.js', '.jsx'],