patchwork/templates: Suppress cycle template tag warning

patchwork/templates: Suppress cycle template tag warning

Running on django 1.7 gives a warning:

RemovedInDjango18Warning: 'The `cycle` template tag is changing to escape its arguments; the non-autoescaping version is deprecated. Load it from the `future` tag library to start using the new behavior.

Although this change doesn't affect this template, suppress the warning
by loading from the future library.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>