Test different versions of django

Author: Emmanouil Konstantinidis

.travis.yml

@@ -8,11 +8,17 @@ python:
   - "pypy"
   - "pypy3"
 
+env:
+  - DJANGO_VERSION=1.7
+  - DJANGO_VERSION=1.8
+  - DJANGO_VERSION=1.9
+
 install:
- - pip install -r requirements.txt
+  - pip install -r requirements.txt
+  - pip install -U django==$DJANGO_VERSION
 
 script:
- - python runtests.py
+  - python runtests.py
 
 notifications:
   slack: