Test user install on Travis

This PR changes Travis setup to test the installed copy instead of the developer build. A corollary to that is the addition of a module which enables users to run `python -m iris.tests.runner` in the same way `python setup.py test` can be run (except it runs on the installed version).
> Originally, I put that in there when I moved the building under script:

OK ... that explains it then.

> Depends if you want all the results even when it fails or not.

Given how long it can take for a travis test to run I'd rather see all the results in one go.