Update django-contrib-comments

Version 1.8.0 of django-contrib-comments moved away from
using the SITE_ID settings attribute in favor of supporting
multi-tenancy.

LEARNER-1583

requirements/base.txt

@@ -5,7 +5,7 @@ django==1.11.2
 django-autocomplete-light==3.1.8
 django-choices==1.4.3
 django-compressor==2.1.1
-django-contrib-comments==1.7.2
+django-contrib-comments==1.8.0
 django-extensions==1.7.8
 django-filter==1.0.4
 django-fsm==2.6.0