various improvements on Puppetmaster  plugin

This pull request does three distinct things:

 * use `/usr/bin/ruby` as an interpreter instead of `/bin/env`, which doesn't exist in Debian (it's in /usr/bin/env) - and I think it's fair to assume that /usr/bin/ruby exists
 * allow the date format to be overriden *and* use the default one that is used in Debian
 * clarify that compile time is in seconds