Pin django-taggit version. (#322)

`django-taggit` 1.3.0 dropped today, breaking a whole lot of Wagtail stuff. This should fix it by keeping to version 1.2.0. Fixes #321.

setup.py

@@ -47,6 +47,7 @@ setup(
         'django-eventtools==1.0.*',
         'django-bootstrap4',
         'Django>2.1,<3.1',              # should be the same as wagtail
+        'django-taggit<1.3',
         'geocoder>=1.38.1,<2.0',
         'icalendar==4.0.*',
         'wagtail==2.8.*',