Prepare enyo for 2.3.1 work

In master branch, update enyo version to
2.3.1-pre.1 and move CHANGELOG notes into 2.3.1-pre.1
section.

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)
Merge pull request #588 from enyojs/start-2.3.1-work

Prepare enyo for 2.3.1 work