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).
No, I ran a [test](https://travis-ci.org/QuLogic/travis-test/builds/30811409) and all those paths are already searched. What it does fix is a not-found error with `libmo_unpack.so` when running the tests.