Change next release to be 2.4.0 instead of 2.3.1

We've decided that post-2.3.0 API changes will be 2.4.0 instead of
2.3.1 to preserve the 2.3.1 naming for maintaince of the 2.3.0
feature set, so CHANGELOG is updated to indicate this.

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)