The version checking code fails on Python 2.6 and earlier.
`sys.version_info` only became a named tuple in Python 2.7. The
recommended way to get the check to work in earlier versions of Python
is to access it as a regular tuple.
Add support for YAML format configuration files. In addition, this
commit adds instructions about YAML config files to the README, and it
also changes the README to encourage use of YAML instead of JSON.
