[ITK] Required CMake version bump to 2.8.9

[Matt McCormick]

Hi,

We are looking at upgrading GDCM, which has a CMake required version
of 2.8.9.  We currently have a CMake minimum required version of
2.8.8. Debian 7.0 (wheezy), which is fairly old, has 2.8.9. Is anyone
using CMake < 2.8.9?

Thanks,
Matt