Only run Facter::Memory.meminfo_number on supported platforms

Having this code outside of the setcode block means it gets evaluated and
generates a spurious warning on unsupported platforms.
Merge branch 'pcarlisle-fix-freebsd-warning_1.7.x' into 1.7.x

* pcarlisle-fix-freebsd-warning_1.7.x:
  Only run Facter::Memory.meminfo_number on supported platforms