Make Travis happy.
Make Travis happy.

- sudo was deprecated.
- Update dist. sqlite3 included in xenial is recent enough. In trusty it
was outdated and not compatible with Django 2.2.
- Use python 3.6 that should pre-exist in xenial.

jlaunonen/kirppu,jlaunonen/kirppu,jlaunonen/kirppu,jlaunonen/kirppu