Move images into place before processing SASS

As part of the SASS processing we base64 and embed every image found in
the SASS files. In order to do this the images need to be in place
beforehand.

This was causing a bug where the first deploy of an image would cause it
to 404.

gulpfile.babel.js

@@ -164,9 +164,9 @@ gulp.task('default',
     'copy:govuk_template:css',
     'copy:govuk_template:js',
     'copy:govuk_template:error_page',
+    'images',
     'javascripts',
-    'sass',
-    'images'
+    'sass'
   ]
 );