fix(redux-devtools-core): fix umd build (#551)

Author: Methuselah96

packages/redux-devtools-core/webpack.config.umd.js

@@ -9,6 +9,7 @@ module.exports = (env = {}) => ({
   },
   output: {
     library: 'ReduxDevTools',
+    libraryExport: 'default',
     libraryTarget: 'umd',
     path: path.resolve(__dirname, 'umd'),
     filename: env.minimize