Upgrade to 7.4.7

We skipped 7.4.6. Both releases are minor bug fixes. No need to upgrade
unless you're hit by one of them. Release announcements are below:

7.4.6 : http://marc.info/?l=bacula-users&m=148930849926451&q=p2
7.4.7 : http://marc.info/?l=bacula-users&m=148966065313348&q=p2