For #2390, add `subprocess.check_output` to Python versions 2.4-2.6.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19697 b5f69653-2bf5-0310-8201-b0dafb8e1405
Small fix to r19697.

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19698 b5f69653-2bf5-0310-8201-b0dafb8e1405