🐞 Fix version of Wagtail required and improve testing

What's Changed

• upgrading tests and required wagtail version by @ntravis in #26

New Contributors

• @ntravis made their first contribution in #26

Full Changelog: 0.4.1...0.4.2

TravisCI only

minor cleanup

- pip cache auto-config
- generic python 3.6 instead of specific version
- remove the python version requirement from the push since we are only building one version