Be a little more explicit with exclusions.

Author: Sean Bleier

.travis.yml

@@ -6,13 +6,21 @@ python:
   - 3.6
   - 3.7
 env:
-  - DJANGO_VERSION=1.11
-  - DJANGO_VERSION=2.0
-  - DJANGO_VERSION=2.1
+  - DJANGO_VERSION='==1.11'
+  - DJANGO_VERSION='==2.0'
+  - DJANGO_VERSION='==2.1'
 matrix:
   exclude:
     - python: 2.7
-      env: DJANGO_VERSION='>=2.0'
+      env: DJANGO_VERSION='==2.0'
+    - python: 2.7
+      env: DJANGO_VERSION='==2.1'
+    - python: 3.5
+      env: DJANGO_VERSION='==1.11'
+    - python: 3.6
+      env: DJANGO_VERSION='==1.11'
+    - python: 3.7
+      env: DJANGO_VERSION='==1.11'
 # command to run tests
 install: ./install_redis.sh
 script: make test DJANGO_VERSION=$DJANGO_VERSION