Run tests against Django 2.0

https://www.djangoproject.com/weblog/2017/dec/02/django-20-released/

Author: michael-k

.travis.yml

@@ -7,6 +7,7 @@ env:
   - DJANGO=1.8.* DRF=3.6.*
   - DJANGO=1.10.* DRF=3.7.*
   - DJANGO=1.11.* DRF=3.7.*
+  - DJANGO=2.0.* DRF=3.7.*
 install:
   - pip install -q Django==$DJANGO djangorestframework==$DRF
   - pip install coveralls
@@ -17,3 +18,7 @@ after_success:
   - coveralls
 before_script:
   - flake8 . --config=flake8
+matrix:
+  exclude:
+    - python: "2.7"
+      env: DJANGO=2.0.* DRF=3.7.*