Improve test coverage

[chris48s]

I've started adding some tests for this package but our coverage level is still low. There's a few problems to tackle:

• In order to write tests for https://github.com/DemocracyClub/dc_signup_form/blob/master/dc_signup_form/views.py we need to create a stub django project for the tests to run in. With the minimal stub app defined in https://github.com/DemocracyClub/dc_signup_form/blob/master/run_tests.py it falls over as soon as you try to render a template because it is expecting stuff like static files, base.html template, dc_base_theme, context processors + so on to exist, and they don't. done in 07d5b1f

• At the moment the tests run against whatever postgres version is current on travis, but the code contains special cases to support Postgres < 9.4 and >= 9.4 so the build config should declare environments which allow us run the tests on multiple postgres versions to properly test that support. added in 0c784fa

• Quite a lot of the untested code is making calls to external APIs so there is some work do on mocking out the interactions with other services (and perhaps some refactoring to make that a bit easier to do)