Fixed error on Debian: "OSfamily Debian not supported."

When using this module on Debian 6.0 (Squeeze) with Puppet 2.7.18, I get this error: "OSfamily Debian not supported."

The code is case-ing on $::osfamily but using $::operatingsystem in the error message. Since the error message correctly had "Debian", I just switched it to case off of $::operatingsystem instead.