Factor.Util.Resolution.exec no longer handles scripts.

This commit prefixes the file path in script_parser with a shell (sh).  The reason this is  now required is that Factor now searches for a binary before executing the command in [Factor.Util.Resolution.exec](http://downloads.puppetlabs.com/facter/apidocs/classes/Facter/Util/Resolution.html#M000070) and if the binary doesn't exist, nil is returned.