Exclude Django 2.1 and 2.2 from Python 2.7 test matrix

.travis.yml

@@ -15,6 +15,10 @@ matrix:
   exclude:
   - python: 2.7
     env: DJANGO="django>=2.0,<2.1"
+  - python: 2.7
+    env: DJANGO="django>=2.1,<2.2"
+  - python: 2.7
+    env: DJANGO="django>=2.2,<2.3"
 before_install:
 - pip install codecov
 install: