Enable transform-react-constant-elements babel on prod

It seems that the issues have been resolved, so this plugin can be used again

Author: SpaceK33z

packages/babel-preset-react-cy-app/index.js

@@ -81,14 +81,9 @@ if (env === 'test') {
   };
 
   if (env === 'production') {
-    // Optimization: hoist JSX that never changes out of render()
-    // Disabled because of issues:
-    // * https://github.com/facebookincubator/create-react-app/issues/525
-    // * https://phabricator.babeljs.io/search/query/pCNlnC2xzwzx/
-    // * https://github.com/babel/babel/issues/4516
-    // TODO: Enable again when these issues are resolved.
-    // plugins.push.apply(plugins, [
-    //   require.resolve('babel-plugin-transform-react-constant-elements')
-    // ]);
+    plugins.push.apply(plugins, [
+      // Reduces calls to `React.createElement` by hoisting static components to the top
+      require.resolve('babel-plugin-transform-react-constant-elements')
+    ]);
   }
 }