Use tar-ustar format for dist

This is required to support file names longuer than 99 characters
when generating the dist tarball, which we require because of the
sphinx theme files in the python bindings documentation.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>