`enhanceLunr` function triggers syntax error on IE11

[abouthalf]

From the Gatsby documentation

Like all gatsby-* files, the code is not processed by Babel. If you want to use JavaScript syntax which isn’t supported by your version of Node.js, you can place the files in a src subfolder and build them to the plugin folder root.

The file commons.js uses ES6 syntax - which is bundled in gatsby-browser.js, but not transpiled by WebPack / Babel - causing failures in IE11