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...
Makes IndexedBase consistent with Symbol wrt to latex printing
Adds test extra test for latex of IndexedBase
Merge pull request #7354 from bjodah/fix_latex_IndexedBase_gamma

Makes IndexedBase consistent with Symbol wrt to latex printing
Merge pull request #7301 from asmeurer/python3.4

Add Python 3.4 to .travis.yml