ENH: Change required ITK version from 4.5 to 4.6

Starting with r23373 ITK 4.6.x is used to build and test Slicer.

$ git shortlog c127db8..6b5fa1a SuperBuild/External_ITKv4.cmake
jcfr (8):
      ENH: Update (1) ITKv4 from v4.5.2 to v4.6rc01 and (2) update BRAINSTools
      BUG: Update ITKv4 project to backport fix related to GDCMIO
      BUG: Update ITKv4 - Attempt to fix ResampleDTI tests
      COMP: Update ITKv4 - Fix "unused-local-typedefs" warning
      COMP: Move most of find_package calls into each Slicer sub-project.
      ENH: Move ITKv4 towards 4.6.0rc20140719
      ENH: Update ITKv4 and BRAINSTools
      BUG: Update ITKv4 and BRAINSTools - BRAINSFit performance improvement.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23661 3bd1e089-480b-0410-8dfb-8563597acbee