Add DJANGO_VERSION 2.2.x into travis config.

Author: zbohm

.travis.yml

@@ -6,6 +6,7 @@ env:
   - DJANGO_VERSION=https://github.com/django/django/archive/stable/1.11.x.zip
   - DJANGO_VERSION=https://github.com/django/django/archive/stable/2.0.x.zip
   - DJANGO_VERSION=https://github.com/django/django/archive/stable/2.1.x.zip
+  - DJANGO_VERSION=https://github.com/django/django/archive/stable/2.2.x.zip
 python:
   - "2.7"
   - "3.4"