Try installing six to see if that fixes race.
@@ -272,6 +272,9 @@ requirements: virtualenv .sdist-requirements
# new version of requests) which we cant resolve at this moment
$(VIRTUALENV_DIR)/bin/pip install "prance==0.6.1"
+ # Fix for Travis CI race
+ $(VIRTUALENV_DIR)/bin/pip install "six==1.11.0"
+
.PHONY: virtualenv
# Note: We always want to update virtualenv/bin/activate file to make sure
# PYTHONPATH is up to date and to avoid caching issues on Travis
