COMP: Update VTK to configure a more friendly VTKConfigVersion file

// -------------------
commit 15fac7625d2b817aee4326376871f2cc2bbf53cd
Author: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>
Date:   Wed Jul 30 06:51:28 2014 -0400

    Configure VTKConfigVersion consistent with VTK's compatibility guarantees

    Since the minimum CMake version required by VTK is 2.8.8, this commit
    uses the module CMakePackageConfigHelpers available in that version of
    CMake.
// -------------------

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23515 3bd1e089-480b-0410-8dfb-8563597acbee