Fix diff command in puppet 2.7

Without this patch, things like 'puppet catalog diff' fail on 2.7.x
(my test environment is 2.7.20) with this error:





(unmodified)
>err: undefined method `execute' for Puppet::Util::Diff:Module
>err: Try 'puppet help catalog diff' for usage

(If I use the Puppet 3.x version)
>>>err: undefined method `execute' for Puppet::Util::Execution:Module
>>>err: Try 'puppet help catalog diff' for usage