Incompatibility with python 2.6

[Max Fischer]

python-crontab uses `collections.OrderedDict`, which is only available since python 2.7 [1]. Trying to install or run python-crontab fails with the following traceback:

    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/private/tmp/pip-build-rWQvdP/python-crontab/setup.py", line 20, in <module>
        from crontab import __version__, __pkgname__
      File "crontab.py", line 94, in <module>
        from collections import OrderedDict

[1]
https://docs.python.org/2.7/library/collections.html#collections.OrderedDict

[Martin Owens (doctormo)]

I've put a very basic ordereddict fallback for python2.6, but tests show that there are still holes in python2.6 support. If you can provide a more robust patch, I'd be willing to take it, but for now python-crontab must be a python2.7 supported module.