Update travis.yml (multiple django envs)

.travis.yml

@@ -4,6 +4,12 @@ python:
     - 3.4
     - 3.5
     - 3.6
+env:
+    - DJANGO='django>=1.8,<1.9'
+    - DJANGO='django>=1.9,<1.10'
+    - DJANGO='django>=1.10,<1.11'
+    - DJANGO='django>=1.11,<2.0'
+    - DJANGO='django>=2.0'
 
 install:
     - travis_retry pip install -r requirements.txt