How to Re-Root a HTC One M8 after Marshmallow ?

[AC Question]

Hi guys,

I recently flashed the Viper MArshmallow rom onto my M8. I didn't realise I was supposed to unroot first, and so as a result, I now have a broken root. I tried to reflash SuperSu but got stuck on bootloop, and had to reflash the rom again. I've looked for two days for the answer to how to deal with this but everyone seems to be using a nexus or windows, and I'm operating with a mac.

So, to recap :-

Rooted previously
Marshmallow Rom installed,
Broken Root

Using a mac to try and re-root in some way.

Thanks in advance for your help guys