Fix empty requires array bug, allow use of built in PPDs

This code change accomplishes two things : 

1. Bug fix - If you choose to not require a driver package, the pkginfo file was creating a requires array with an empty string.  This created a requirement that Munki could not resolve.  This was fixed by adding a conditional when building the makepkginfo command (lines 422-425)

2. Enhancement - I have some printers that use the built in Generic PCL driver.
      - using /usr/sbin/lpinfo -m  will list installed PPDs as well as the built in ones (line 192)
      - to use the built in PPDs the lpadmin command uses -m instead of -p  (lines 356-362)