Test against multiple versions of Oscar and Django

To anticipate migrating to Oscar 1.1 and Django 1.8, let's run the
tests on Travis already.

Author: maiksprenger

.travis.yml

@@ -4,8 +4,13 @@ python:
   - "3.3"
   - "3.4"
 
+env:
+  - DJANGO=Django==1.7.8 OSCAR=django-oscar==1.0.2
+  - DJANGO=Django==1.7.8 OSCAR=django-oscar==1.1
+  - DJANGO=Django==1.8.2 OSCAR=django-oscar==1.1
+
 install:
-  - pip install 'django-oscar>=0.7'
+  - pip install $DJANGO $OSCAR
   - pip install -r requirements.txt
 
 env: