after you have completed the DB quest and you get the dungeon, keep casting soul trap on one of the prisoners to level up your conjuration.

Have they patched being able to cast it on dead bodies?

I dont think so.
I recently used a dead mudcrab to get my conjuration from 87 to 90.
I think this was after patch 1.5, but I am not 100% on that.
Maybe it was the day before.

I've got 1.5 and used it on a dead walrus for a few levels tonight. So it still works.

You can do this on any corpse. Mudcrabs, necromancers, winterhold guards etc. I don't think the corpse affects the amount of XP you get from soul trap either so you should be gold.