Try a workaround for travis build failure issue.
@@ -381,6 +381,9 @@ requirements: virtualenv .sdist-requirements install-runners
# Fix for Travis CI race
$(VIRTUALENV_DIR)/bin/pip install "six==1.11.0"
+ # Fix for Travis CI caching issue
+ $(VIRTUALENV_DIR)/bin/pip uninstalli -y "pytz"
+
# Install requirements
#
for req in $(REQUIREMENTS); do \
