Disallow "version" as a field inside of dependency objects

[AlexIIL]

Since the correct field name is versions - and I think we want to allow plugins to add their own fields. This should help find issues such as https://github.com/henkelmax/simple-voice-chat/pull/419.

(However we could make plugins prefix their custom fields with their ID, so minecraft:environment might be the field name of side restrictions. Technically this is a problem we should solve separately though).