ENH: Use the default ExternalProject UPDATE_COMMAND for ITKv4.

As we are rapid corresponding changes to the ITK repository for the Software
Guide, the Superbuild ITKv4 build should automatically update master on each
build.  Remove the old update command and required the latest version of CMake
for the best ExternalProject.cmake script.

Change-Id: Ie362128bb35d5a550f31f3bc387d74343cfb1710
Merge "ENH: Use the default ExternalProject UPDATE_COMMAND for ITKv4."