Install certifi when testing against Elasticsearch 5

Required by elasticsearch-py but needs to be installed separately

8314f7b25c

tox.ini

@@ -33,6 +33,7 @@ deps =
     elasticsearch: elasticsearch>=1,<2
     elasticsearch2: elasticsearch>=2,<3
     elasticsearch5: elasticsearch>=5,<6
+    elasticsearch5: certifi
 
 setenv =
     postgres: DATABASE_ENGINE=django.db.backends.postgresql_psycopg2