Update VE core submodule to master (a2709b5)

New changes:
0516906 ve.ce.Surface: Prevent incorrect pawning in Opera 12
8e68032 build: Merge intro.css and intro.js.txt to intro.txt as they're identical
6166717 Update OOjs UI to v0.1.0-pre (85cfc2e735)

Change-Id: Ia91d7b2fe09f2b23ad55de3bbb3d74a8644ffc94