Update .travis.yml to exclude incompatible versions

.travis.yml

@@ -1,23 +1,34 @@
 language: python
 sudo: false
+
 python:
  - "2.7"
  - "3.4"
  - "3.5"
  - "3.6"
+
 env:
  - DJANGO_VERSION=1.10.8
  - DJANGO_VERSION=1.11.14
  - DJANGO_VERSION=2.0.7
+
+matrix:
+  exclude:
+    - python: 2.7
+      env: DJANGO_VERSION=2.0.7
+
 install:
  - pip install -q -r requirements/base.txt
  - pip install -q -r requirements/tests.txt
  - pip install -q django==$DJANGO_VERSION
  - pip install coveralls
+
 before_script:
  - createdb -E UTF-8 dash -U postgres -O $USER
+
 script:
  - coverage run manage.py test --settings=test_runner.settings_travis --verbosity=2 --noinput
  - flake8
+
 after_success:
  - coveralls