Fix travis build, pip no longer support use-mirrors option.

Author: jbub

.travis.yml

@@ -6,5 +6,5 @@ python:
   - "2.7"
   - "3.3"
   - "3.4"
-install: "pip install -r dev-requirements.txt --use-mirrors"
+install: "pip install -r dev-requirements.txt"
 script: make test