Install a compatible version of setuptools on Python 2.6.
Install a compatible version of setuptools on Python 2.6.

Setuptools dropped Python 2.6 support back in 36.8.0, but up until
fairly recently it still worked. Now that it's definitely off of Python
2.6, we want to make sure we don't accidentally install a newer version
than that when building the package.

Testing Done:
Successfully ran `setup.py develop` on Python 2.6 and 2.7.

Reviewed at https://reviews.reviewboard.org/r/9529/

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