Re-enables Python 2.6 for ReproUnzip

Python 2.6 was added with d12c20a3. It was later removed by b60fb781 when I found [Python bug 13676](http://bugs.python.org/issue13676). da39940f added warnings, and 634ca22c turned both warnings into errors (exiting with 1 if running <2.7.3) after [#60](https://github.com/ViDA-NYU/reprozip/issues/60#issuecomment-54715548).

This allows reprounzip to run on Python 2.6 again, and tests it. During these tests, "reprounzip graph" is skipped, and Python 2.7 is used for reprozip (only reprounzip can possibly work on <2.7.3).