Add future parser support on puppet::agent

Documentation aside, future parser appears to need agent-side enabling based on my experiments using puppetdbquery / puppet-kick. This defaults to current parser, so no behavior will change.
Based on the [readme in the puppet 4.0 language specification](https://github.com/puppetlabs/puppet-specifications), I think that that once we get to puppet 4 we'll be switching to referencing numeric language versions rather than using the current/future terminology.