BUG: Correct CMake variable passed to SimpleITK build

The depricated PYTHON_INCLUDE_PATH was being set which is not handled
in SimpleITK build. This has been corrected to the newer
PYTHON_INCLUDE_DIR.
Merge pull request #179 from blowekamp/FixSimpleITKCMakeInclude

BUG: Correct CMake variable passed to SimpleITK build