Run travis with speedups enabled and disabled

This modifies the Travis build to run the entire test suite with speedups both enabled and disabled - removing any special code to test speedups. The goal is to catch issues like #150 before merging, and enforce speedups parity end to end. Downside is it doubles the number of Travis runs.

It exists on top of master so PR #152 is not included but it catches that. It also exposes a couple of other existing (and unfiled) bugs too - which is why the tests [here](https://travis-ci.org/jwass/Shapely/builds/33091981) are failing.

I would have liked to pass in the speedups parameter on the command line to `python setup.py test` but distutils doesn't document this very well, so it's just done via environment variable.
@pelson - I see. You're right, I'll make sure that if we expect the speedups to run and they're unavailable or fail to load, we'll bail and report that up to Travis.