Log Message:
py-django-countries: updated to 5.1.1

5.1.1:
Fix some translations that were included in 5.1 but not compiled.

5.1:
Tests now also cover Django Rest Framework 3.7 and Django 2.0.
Allow for creating country fields using (valid) alpha-3 or numeric codes.
Fix migration error with blank default.
Add a {% get_countries %} template tag.