Add Geo tracking DB rollback CI step

Rolls back the latest possible migration.

.gitlab-ci.yml

@@ -636,6 +636,13 @@ db:rollback-mysql:
   <<: *db-rollback
   <<: *use-mysql
 
+db:rollback-pg-geo: &db-rollback
+  <<: *db-rollback
+  <<: *use-pg
+  script:
+    - bundle exec rake geo:db:migrate VERSION=20170627195211
+    - bundle exec rake geo:db:migrate
+
 gitlab:setup-pg:
   <<: *gitlab-setup
   <<: *use-pg