Improve alert behaviour on backup age

- Use mtime instead of ctime, this gives the date the backup was
  modified on the *source* box rather than when it was copied onto the
  backups box.

- Allow 1/8th of a day tolerance to allow for the time between the
  backup being created and it being copied over.
Merge pull request #337 from alphagov/improve-backup-alert-time-checking

Improve alert behaviour on backup age