The client's boot script supports older browsers than the rest of the client so
it can log a warning if it can't start in "somewhat" older browsers.

Update the boot script's supported officially browsers from ~2013 (IE 11) to
~2017 (Safari 11), although the code will probably still run in older browsers.
This allows dropping an async/await Babel plugin, although async is not actually
used in the current boot script.