Add an explicit dependency on rubygem-ffi

Facter is able to operate without FFI support, but some FreeBSD facts depend on
FFI to be fetched, so add an explicit dependency in order to make sure the
following facts are resolvable:

• disks
• dmi
• load_averages
• memory.system
• partitions
• processors.count
• processors.models
• processors.speed

Bump PORTREVISION.

With hat: puppet