Updated CI configuration

As of pip 7.0.0 —use-mirrors has been removed.

.travis.yml

@@ -12,7 +12,7 @@ env:
     - DJANGO="1.8.13"
 
 install:
-    - pip install Django==$DJANGO --use-mirrors
+    - pip install Django==$DJANGO
     - pip install coverage==3.6
     - pip install python-coveralls==2.4.0
     - export PYTHONPATH=.