Just hardcoding to react_app_ga

putting process.env into html gives parsing error, going to just
hardcode for now. Will either eject from cra or bring in their fix from
facebook#1037

Author: lyleg

Diff for: packages/react-scripts/config/webpack.config.prod.js

@@ -218,7 +218,10 @@ module.exports = {
     // <link rel="shortcut icon" href="%PUBLIC_URL%/favicon.ico">
     // In production, it will be an empty string unless you specify "homepage"
     // in `package.json`, in which case it will be the pathname of that URL.
-    new InterpolateHtmlPlugin(env['process.env']),
+    new InterpolateHtmlPlugin({
+      PUBLIC_URL: publicUrl,
+      REACT_APP_GA: env['process.env'].REACT_APP_GA || ''
+    }),
     // Generates an `index.html` file with the <script> injected.
     new HtmlWebpackPlugin({
       inject: true,