test: remove use-mirrors

.travis.yml

@@ -5,8 +5,8 @@ language: python
 python:
   - "3.5"
 
-# command to install dependencies, e.g. pip install -r requirements.txt --use-mirrors
-install: pip install -r requirements.txt --use-mirrors
+# command to install dependencies, e.g. pip install -r requirements.txt
+install: pip install -r requirements.txt
 
 # command to run tests, e.g. python setup.py test
 script: python setup.py test