v1.2.1 for Minecraft 1.9, Forge 12.16.1.1887

Fixes a minor issue with low-power enchants introduced in v1.2.

When there are sufficiently few bookshelves, the power level can be low enough that no enchant is selected for a slot. The mod had a regression where this wasn't handled properly. Older versions of Minecraft (1.10.2 and below) still let you click on these empty slots, and doing that will confuse the mod temporarily, but the fact that the empty slot exists is now handled properly.

Also, added update.json tracking to the mod, so future releases will be notified in-game.