fix build for new django-polymorphic 1.3

this version no longer supports Django 1.7

Author: vdboor

.travis.yml

@@ -7,7 +7,7 @@ python:
 - '3.5'
 - '3.6'
 env:
-- PACKAGES="django>=1.7,<1.8 django-mptt<0.8"
+- PACKAGES="django>=1.7,<1.8 django-mptt<0.8 django-polymorphic<1.3"
 - PACKAGES="django>=1.8,<1.9"
 - PACKAGES="django>=1.9,<1.10"
 - PACKAGES="django>=1.10,<1.11"
@@ -18,9 +18,9 @@ matrix:
   - python: '3.3'
     env: PACKAGES="django>=1.10,<1.11"
   - python: '3.5'
-    env: PACKAGES="django>=1.7,<1.8 django-mptt<0.8"
+    env: PACKAGES="django>=1.7,<1.8 django-mptt<0.8 django-polymorphic<1.3"
   - python: '3.6'
-    env: PACKAGES="django>=1.7,<1.8 django-mptt<0.8"
+    env: PACKAGES="django>=1.7,<1.8 django-mptt<0.8 django-polymorphic<1.3"
 before_install:
 - pip install codecov
 install:

tox.ini

@@ -9,15 +9,18 @@ envlist=
 
 [testenv]
 deps =
-    django-polymorphic >= 0.8.1
     django17: Django >= 1.7, < 1.8
     django17: django-mptt < 0.8.0
+    django17: django-polymorphic < 1.3
     django18: Django >= 1.8, < 1.9
     django18: django-mptt >= 0.8.0
+    django18: django-polymorphic >= 1.3
     django19: Django >= 1.9, < 1.10
     django19: django-mptt >= 0.8.0
+    django19: django-polymorphic >= 1.3
     django110: Django >= 1.10, < 1.11
     django110: django-mptt >= 0.8.6
+    django110: django-polymorphic >= 1.3
     py26: mock
     py27: mock
 ;    django-dev: https://github.com/django/django/tarball/master