install coverage before launch

.travis.yml

@@ -20,7 +20,7 @@ script:
 install:
  - pip install -r requirements.txt
  - pip install -r stress_tests/requirements.txt
- - pip install yapf
+ - pip install yapf nose2 mock coverage nose2[coverage_plugin]
 after_success:
  - coverage xml
  - ./cc-test-reporter after-build -t coverage.py --exit-code $TRAVIS_TEST_RESULT