COMP: Fix failing header test

Disable this test if Python version is less than 2.6.

The fixed error:
  File "/scratch/dashboards/Linux-x86_64-gcc4.1/ITK/Utilities/Maintenance/BuildHeaderTest.py", line 21
    from __future__ import print_function
SyntaxError: future feature print_function is not defined

Change-Id: I14c525e3aff46b78edcf9d2690d8bf0bb2b29061
Merge topic 'HeadertestPython2.6'

0ff8be3d COMP: Fix failing header test