Refactor test running into one source of truth

- In development run `./run_tests.sh`
- `jenkins.sh` uses `./run_tests.sh`
- `.travis` uses `./run_tests.sh`

Simples!

This assumes that you're using virtualenvwrapper in development (or at
least have your `WORKON_HOME` environment variable set). If you're not
then you will need to be in a virtualenv before executing
`./run_tests.sh`
Merge branch 'master' into refactor-run-tests
Ensure that pep8 errors make the test script fail

@maxfliri
Merge pull request #114 from alphagov/refactor-run-tests

Refactor run tests
Restore creation of coverage report

@maxfliri, @gtrogers