use upstream release-8.0 in travis as a workaround for adal requirement

Author: roycaihw

run_tox.sh

@@ -36,6 +36,7 @@ popd > /dev/null
 cd "${TMP_DIR}"
 git clone https://github.com/kubernetes-client/python.git
 cd python
+git checkout release-8.0
 git config user.email "[email protected]"
 git config user.name "kubenetes client"
 git rm -rf kubernetes/base