- Drops support for anything < 1.7.
- Use `django.apps.apps.get_app_configs()` for template loaders and
helper discovery.
- Delays environment creation as long as possible, and caches it, should
fix #50.
- Removes direct access to jingo.env, use jingo.get_env() instead.
- Removes env dependency from Registry().
- Moves Template() higher in the module and sets
Environment().template_class directly.
- Adds `django.contrib.admin.apps.SimpleAdminConfig` to test settings
to ensure support for AppConfig classes, fixes #68.
64402d2fcc