Fix travis

was failing with
    no such option: --use-mirrors

Author: bmwiedemann

.travis.yml

@@ -5,7 +5,7 @@ python:
     - 2.7
 
 install:
-    - pip install -r requirements_dev.txt --use-mirrors 
+    - pip install -r requirements_dev.txt
     - python setup.py install
 
 script: python setup.py test