.travis.yml: avoid Python 3.2

"pip install" fails on Python 3.2 because simplejson does
not support Python < 3.3, and simplejson is used by pip.

Avoid travis build failures by excluding Python 3.2 from
the Travis CI builds.

jsonpickle itself *does* support Python 3.2 so setup.py
is intentionally left as-is.

Signed-off-by: David Aguilar <davvid@gmail.com>