Update VE core submodule to master (2a2ebe9)

New changes:
ca2d078 Grunt: Create a distribution files for the ve module
ff4bf36 Run node-svgo to crush all SVGs whilst retaining readibility
076edd4 Provide the (currently-unused) external link icon in LTR and RTL variants
96ff968 Use globalOverlay as the contextOverlay in MobileContext
374fd68 In the toolbar, display superscript tool above subscript tool
e800343 Update OOjs UI to v0.1.0-pre (432a76f7d4)
2d4b557 Use CSSJanus to generate RTL stylesheets

Change-Id: If07518c1a7b72fb0fadd7ee840809006ad87a3c0