travis: install yapf

.travis.yml

@@ -13,6 +13,7 @@ script:
 install:
  - pip install -r requirements.txt
  - pip install -r stress_tests/requirements.txt
+ - pip install yapf
  - pip install --force-reinstall "coverage>=4.0,<4.4" codeclimate-test-reporter
 after_success:
  - codeclimate-test-reporter