remove deprecated --use-mirrors in travis config

Author: abooker

.travis.yml

@@ -4,8 +4,8 @@ python:
   - "2.7"
 # command to install dependencies
 install:
-  - "pip install -r requirements.txt --use-mirrors"
-  - pip install coveralls --use-mirrors
+  - "pip install -r requirements.txt"
+  - pip install coveralls
 # command to run tests
 script:
 - "nosetests --with-coverage --cover-erase --cover-branches --cover-package=the_ark"