Fix case of stage environment variable. (#1014)

Author: microbit-matt-hillsdon

public/index.html

@@ -18,7 +18,7 @@
       content="A Python Editor for the BBC micro:bit, built by the Micro:bit Educational Foundation and the global Python Community."
     />
     <% if (process.env.REACT_APP_GA_MEASUREMENT_ID &&
-    (process.env.REACT_APP_STAGE === 'production' || process.env.REACT_APP_STAGE
+    (process.env.REACT_APP_STAGE === 'PRODUCTION' || process.env.REACT_APP_STAGE
     === "STAGING")) { %>
     <script
       async