Update vulture from 0.27 to 0.28

Author: pyup-bot

dev-requirements.txt

@@ -19,5 +19,5 @@ pytest-cov==2.5.1  # Measure code coverage
 pytest-envfiles==0.1.0  # Use files for environment variables for tests
 pytest==3.6.2  # Test runners
 timeout-decorator==0.4.0  # Decorate functions to time out.
-vulture==0.27
+vulture==0.28
 yapf==0.21.0  # Automatic formatting for Python