Release v2.5.0

Time for a new release! :sparkles: 

Thinking about going back and improving #3032 to handle Hashes better (or at all) and getting #2865 into 2.5. But figured I'd get this release stuff started anyway.

- [x] Release post
- [x] Update history & version file (`rake site:generate`)
- [x] Handle hashes in `jsonify` filter (#3063)
- [x] Require `:jekyll` bundler group on start-up. (#2865)