Update .travis.yml to use Python 3.

* Python 3 and Django < 1.5 are incompatible, so we'll
  need to exclude those combinations.
Use python3-openid and fix deprecations.

* openid isn't compatible with P3, but some merciful soul
  has made a version of it that works. Change setup.py
  to install the correct version (and only the correct
  version).
* test_auth uses assertEquals a bunch of times; change
  that to the non-deprecated assertEqual.
Base64 and openid compatibility tweaks.

* encodestring and decodestring are outdated, so swap them for
  b64encode and b64decode.
* python-openid and python3-openid have a different name for
  [get]ExpiresIn, so we need to do some hacking to ensure
  compatibility.