Thanks for your patience everyone. I have finally updated Armor Stand Tools for 1.9. Here's the changelog:

- Updated for Spigot/Craftbukkit 1.9 (Backwards compatible with 1.8.x)
- Added GUI slot for setting the item in the Armor Stand's left hand (1.9 only)
- Added config.yml option for default item in left hand (inOffHand)
- Workaround for setting player skulls in 1.9

I have tested everything as thoroughly as I can, but let me know on this thread asap if you come across a bug. Cheers.