Update VE core submodule to master (5003907)

New changes:
7b68782 Update OOjs UI to v0.1.0-pre (3b434d5388)
ecb194d Remove border-radius prefixes
993220a Fix invalid use of border shorthand syntax
580473e build: Clean up repo split left overs in Gruntfile

Incidental change:
* The OOjs UI-provided 'autoAdd' configuration option for tools has been
  replaced with 'autoAddToCatchall' and 'autoAddToGroup'.

Change-Id: Ie7646ae867e5c6ca616c3f9045c79b886e78475c