ignore lines not starting with 'printer' in lpstat -p

sometimes the output of 'lpstat -p' contains lines like 'Printer is now
on-line' or 'Ready to print'. In these cases facter (or a puppet run)
outputs the following warning:

Could not retrieve printers: undefined method `captures' for nil:NilClass

This patch ignores lines not starting with 'printers'
capture printer name also when it is printing

when a printer is printing the lpstat output is:
'printer name now printing'
and the previous regexp would skip the printer
Merge pull request #24 from leinaddm/fix_regexp

fix nil and regexp