ENH: Update superbuild to ITK 4.13.1 so that it builds with gcc 8
The current version of ITK in the Superbuild (4.12.0) does not build with gcc 8 because ITK/Modules/ThirdParty/VNL/src/vxl/vcl/vcl_compiler.h contains hard coded compiler versions and is not up to date. This has been corrected in 4.13.1.