Update VE core submodule to master (31fb320)

New changes:
ccdb9db Update OOjs UI to v0.1.0-pre (93f94e059f)
191d9d8 jsduck: Have a default --meta-tags setting
6f9b262 CONTRIBUTING: Refer to MediaWiki conventions for jsduck

Change-Id: Ibb135f587e35ef6751759faac5b3372153ad6c82