Ensure virtualenv is upgraded when installing tox. Fixes jaraco/path#188.
@@ -8,7 +8,8 @@ python:
cache: pip
install:
-- pip install tox
+# ensure virtualenv is upgraded to avoid issues like jaraco/path#188
+- pip install -U --upgrade-strategy=eager tox
before_script:
# Enable IPv6. Ref travis-ci/travis-ci#8361
