Speed up Travis build times

I'm working on a strategy to use `tox` more selectively with Travis to speed up builds.

`tox` is great for local dev where the tox environ gets set up once and is re-used but setting up 6 environments (not including `logs` and `dist`) is a large cost.

```
$ ls .tox/
cover/  dist/  docs/  lint/  log/  py26/  py27/  regression/
$ ls .tox/log/
tox-0.log
$ ls .tox/dist/
gcloud-0.02.2.zip
```
