Stop using Babel to transpile gulpfile

Bumping NodeJS to 10.15.3 removes the need to
transpile `gulpfile.js` as this version includes
all the ES6 features used..

Also removes .babelrc file. This was included to
ensure the gulpfile was transpiled correctly so it
is no longer needed.

.babelrc

@@ -1,3 +0,0 @@
-{
-  "presets": ["es2015"]
-}

gulpfile.js

@@ -138,7 +138,7 @@ const watchForChanges = () => {
   return watch(paths.src + 'javascripts/**/*', ['javascripts'])
     .watch(paths.src + 'stylesheets/**/*', ['sass'])
     .watch(paths.src + 'images/**/*', ['images'])
-    .watch('gulpfile.babel.js', ['default']);
+    .watch('gulpfile.js', ['default']);
 };
 
 const lint = {