Force `rm` to delete files w/o confirmation

This commit adds the `-f` switch to `rm` in order to force removal of files
with an mtime >30 days. Without the `-f` flag, the command prompts - if this
were happening automatically in a cron job, files would never be removed as
no confirmation would be given automatically.
Merge pull request #52 from alphagov/change-archive-cmd-force

Force `rm` to delete files w/o confirmation