Use json to encode invalid values in configuration errors so that the user sees a proper repr of the value.

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

script/travis/render-bintray-config.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
-from __future__ import print_function
 from __future__ import absolute_import
+from __future__ import print_function
 from __future__ import unicode_literals
 
 import datetime