add Travis configuration

Created by: rjw57

This PR adds a Travis configuration to run tests in Djangos 1.8, 1.11 and 2.0 with Pythons 2.7, 3.4, 3.5 and 3.6.

The combination of Django 2.0 and Python 2.7 is excluded since Django 2.0 no longer supports Python 2.7

Commits 3877696d and 0ecaba93 provide fixes for issues with earlier Djangos and Pythons which were found when writing this PR. Commit 9bc63f66 provides a missing mock for the Lookup API which is unavailable outside the CUDN. The remaining test suite was tested with no network connection to make sure there aren't any other unexpected calls to network services.