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

script/versions.py

@@ -21,7 +21,9 @@ For example, if the list of versions is:
 `default` would return `1.7.1` and
 `recent -n 3` would return `1.8.0-rc2 1.7.1 1.6.2`
 """
+from __future__ import absolute_import
 from __future__ import print_function
+from __future__ import unicode_literals
 
 import argparse
 import itertools