Update VE core submodule to master (48dd09a)

New changes:
48dd09a Update OOjs UI to v0.1.0-pre (8ac38a5c31)

Add OOjs UI's new Apex theme to the module definition (for users on
older versions of MediaWiki that do not have it available).

Change-Id: Ia79c2380aa2368d58e5980131cf70c91576dfd49