With the recent Steam sale and I think new content release, Darkest Dungeon has been surfacing a bit.

A few users have reported that they can't figure out Darkest Dungeon modding and opening .skel files.
I found this post on the Darkest Dungeon Workshop discussions on steam:
http://steamcommunity.com/groups/dd-workshop/discussions/0/135514800409348324/

The game uses version 2.1.27 so you need to revert your Spine editor to that older version: 2.1.27

• You can do this by going to Spine > Settings... > Version > Other... button.
• And entering 2.1.27.
• Then closing and relaunching Spine.
  

Note that this version is from Apr 2015. Spine: Changelog: v2.1.27
As such, we don't provide updates for that version of the editor anymore, but it should still work.

See the workshop discussion page for more thorough official info. They answered a few more specific questions.
http://steamcommunity.com/groups/dd-workshop/discussions/0/135514800409348324/