Don't install Sphinx on Travis Python 3.3 target

[zaneb]

Sphinx doesn't support Python 3.3, and we already don't run Sphinx in the Travis Python 3.3 target. However, it fails to install because there is no longer any Sphinx package available for Python 3.3. So avoid even trying to install it.

[zaneb]

Yeah, I just noticed there's another PR (#266) to drop support for 3.3 altogether.

[hugovk]

Here's the pip installs for testtools from PyPI for last month, very little for 3.3:

python_version	percent	download_count
2.7	83.99%	105,594
3.5	11.59%	14,566
3.6	2.83%	3,559
3.4	1.43%	1,797
2.6	0.10%	126
3.7	0.04%	48
3.3	0.03%	38

Source: pypinfo --start-date -31 --end-date -2 --percent --pip --markdown testtools pyversion

[rbtcollins]

Merged the 3.3 removal patch