Update travis

.travis.yml

@@ -7,7 +7,7 @@ python:
 env:
     - DJANGO='django>=1.10,<1.11'
     - DJANGO='django>=1.11,<2.0'
-    - DJANGO='django>=2.0'
+    - DJANGO='django>=2.0,<2.2'
 
 install:
     - travis_retry pip install $DJANGO

requirements.txt

-Django>=1.10<2.2; python_version >= '3.4'
+Django>=1.10,<2.2; python_version >= '3.4'
 Pillow>=2.9.0
 apscheduler>=3.3.0
 #django-constance[database]>=2.1