TST: Run CI tests for Python 2.6 on CentOS
Run the CI testing through Python 2.6.
The 2.6 tests run on a CentOS image since Debain packages for 2.6 are scarce, and most of the 2.6 installations still around are probably RedHat based. There was one issue where 2.6 can't run tests using the python -m ase.test
command, and the old versions of numpy and scipy made fire.py fail again, but all tests pass now for all versions.