Force pytest-django verison for testing compatibility with Django 1.6

.travis.yml

@@ -22,7 +22,7 @@ before_install:
 install:
 - |
   if [ "$TEST_TYPE" = build ]; then
-    pip install pytest==3.0.2 pytest-cov pytest-benchmark coveralls six pytest-django mock django-filter
+    pip install pytest==3.0.2 pytest-cov pytest-benchmark coveralls six pytest-django==2.9.1 mock django-filter
     pip install django==$DJANGO_VERSION
     pip install -e .
     python setup.py develop

setup.py

@@ -42,7 +42,7 @@ setup(
     tests_require=[
         'django-filter>=0.10.0',
         'pytest',
-        'pytest-django',
+        'pytest-django==2.9.1',
         'mock',
         # Required for Django postgres fields testing
         'psycopg2',