Work with Python 2.6

Work with Python 2.6

Passing Travis CI build: https://travis-ci.org/msabramo/pundler/builds/26441783

```
$ tox
...
  py26: commands succeeded
  py27: commands succeeded
  py32: commands succeeded
  py33: commands succeeded
  py34: commands succeeded
  pypy: commands succeeded
  congratulations :)
```