add Django 2.0 to Travis tests

.travis.yml

@@ -1,18 +1,25 @@
 language: python
 python:
-  - "2.7"
-  - "3.5"
-  - "3.6"
-  - "pypy"
+  - 2.7
+  - 3.5
+  - 3.6
+  - pypy
 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,<2.1"
 install:
   - pip install PLY
   - pip install -U $DJANGO
   - pip install -e .
 script: python test_project/manage.py test core.tests
 notifications:
   slack: ivelum:m5sGlwz1uEy5sQ32n0kg9lNh
+matrix:
+  exclude:
+    - python: 2.7
+      env: DJANGO="Django>=2.0,<2.1"
+    - python: pypy
+      env: DJANGO="Django>=2.0,<2.1"