match Python 2 minor version on EMR

Starting with the 3.x AMIs, both python2.6 and python2.7 are available. We should try to use whichever one matches our current version of Python.

On the 2.x AMIs, we want to use python2.7 where it's available (2.4.3+) because it comes with pip, and apt-get install -y python-pip no longer works. Prior to that, we should use python2.6.

There's only one Python 3 version available on EMR, so let's stick with python3 for that.