added travis config for both DRF2 and DRF3

.travis.yml

@@ -5,8 +5,14 @@ python:
   - "2.7"
   - "pypy"
 
+env:
+  - "DRF='djangorestframework<3'"
+  - "DRF='djangorestframework>=3'"
+
 # command to install dependencies, e.g. pip install -r requirements.txt --use-mirrors
-install: pip install -r requirements-dev.txt
+install:
+  - pip install $DRF
+  - pip install -r requirements-dev.txt
 
 # command to run tests, e.g. python setup.py test
 script: make check