v2.5
The only change is that the pre-draft-15 compatibility code has been removed.
I recommend to update to this acme4j version soon. Let's Encrypt posted a sunset date for the old GET requests on November 1st 2019. acme4j < v2.5 will stop working after that date. (acme4j v1 is also affected.)
If you use acme4j to connect to a server that does not support ACME draft-15 yet, you should not update to acme4j v2.5 before the server has been updated. The Boulder (Let's Encrypt) and Pebble ACME servers both support draft-15.