Add Python 3.4 to .travis.yml

I don't know if Travis actually supports it yet. I guess we'll find out.

For the slow tests, we only tested on the most recent version of Python 2 and
Python 3, so change them from 3.3 to 3.4.

This is a lot of Pythons now. We should probably think about dropping 3.2
support...
Merge pull request #7301 from asmeurer/python3.4

Add Python 3.4 to .travis.yml