Add Python 2.6 compatibility

The Amazon Linux AMIs have Python 2.6 (.9), not Python 2.7, installed (even the latest versions!). As a result, the Python 2.7 features in `mcv` cause it to be unusable in a CloudFormation stack.

This PR aims to make the code compatible with Python 2.6. Unfortunately, it's difficult for me (with my relative ignorance of the Python ecosystem) to be confident that the code will be fully functional on Python 2.6. I am confident that the code compiles in Python 2.6.9, but that doesn't mean there won't be any issues at run-time.

Also unfortunately, the CloudInit package system installs python packages with `easy_install`, which of course only works for publicly released versions of packages. So I'm not sure how to test this without actually cutting a new version of `mcv`, which isn't ideal. @elliot42 - any ideas? I'll keep trying to figure it out (and will reply here if I do), but just thought you might know something off the top of your head.