Update pre-commit config to enforace that future imports exist in all files.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

script/travis/render-bintray-config.py

@@ -1,5 +1,7 @@
 #!/usr/bin/env python
 from __future__ import print_function
+from __future__ import absolute_import
+from __future__ import unicode_literals
 
 import datetime
 import os.path