Make postrotate send a USR1 rather than upstart

The nginx version that we use in development, staging and production
(1.4.4) doesn't support doing a rotate by upstart, which is different
from the version that we're using in preview (1.5.11).

:(
Merge pull request #396 from alphagov/nginx-json-postrotate-service-rotate

Make postrotate send a USR1 rather than upstart