[AIRFLOW-6766] Fix "cannot import ensure_text" error for pre-commit (#7393)

* [AIRFLOW-6766] Fix "cannot import ensure_text" error for pre-commit

As of today Travis bundles six version 1.11.0 with their python
3.6 image and it misses ensure_text method. Bumping to 1.14+
solves the problem.

scripts/ci/ci_before_install.sh

@@ -33,6 +33,7 @@ KUBERNETES_MODE=${KUBERNETES_MODE:=""}
 
 mkdir -p "${AIRFLOW_SOURCES}/files"
 
-sudo pip install pre-commit
+# We need newer version of six for Travis as they bundle 1.11.0 version
+sudo pip install pre-commit 'six~=1.14'
 
 script_end