COMP: Update minimum required VTK version

This commit will fix the following warning:

CMake Error at CMakeLists.txt:701 (find_package):
  Could not find a configuration file for package "VTK" that is compatible
  with requested version "6.1".

  The following configuration files were considered but not accepted:

    /path/to/Slicer-SuperBuild/VTKv6-build/VTKConfig.cmake, version: 6.2.0

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23514 3bd1e089-480b-0410-8dfb-8563597acbee