Fix things to support Python 2.6.

This is mostly fixes to string formatting to avoid using
empty-bracket-style replacement. This option doesn't work in
Python 2.6 or 3.0. The other major thing is that some 2.7
specific unittest things were used, so unittest2 is added
to install_requires if run under python 2.6.

The other major thing needed to make this work is a fix in
expiringdict for which I've opened issue mailgun/expiringdict#2.
Once that is merged, pushed to PyPI and the requirements updated
in flanker, all tests should pass in Python 2.6.