[travis] Fix not supported option.

.travis.yml

@@ -12,7 +12,7 @@ before_install:
 - git submodule update --init --recursive
 - sudo apt-get update -qq
 - sudo apt-get install -qq doxygen doxygen-latex libboost-all-dev libeigen3-dev
-- sudo pip install cpp-coveralls --use-mirrors
+- sudo pip install cpp-coveralls
 script: ./.travis/build
 language: cpp
 branches: