BUG: USE_ITK_FILE -> ITK_USE_FILE.
Merge pull request #263 from thewtex/itk-use

BUG: USE_ITK_FILE -> ITK_USE_FILE.
BUG: Addressed cmake config errors when TubeTK_USE_SLICER was set
ENH: Use TubeTK-specific var to track downloaded code, not Slicer_SOURCE_DIR
BUG: Reduce CMake version requirement until travis is upgraded.
Merge pull request #265 from aylward/CleanUpCMake

Clean up cmake