BUG: Only use launcher with TubeTK_USE_SLICER.

When run with valgrind during the CTest check, every test gets about 10 false
positives from the shell executable.

Only use the launcher with TubeTK_USE_SLICER.
Merge pull request #204 from thewtex/unix-launcher

BUG: Only use launcher with TubeTK_USE_SLICER