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).
Things in `script:` will run even if previous commands fail. This option makes it fail early. Originally, I put that in there when I moved the building under `script:`, but I've since moved that back under `install:`, so now it's optional, really. Depends if you want all the results even when it fails or not.