(PUP-2093) Migrate all PMT code to use JSON.

Prior to this commit, there were scattered bits throughout the PMT code
that still lived under the tyranny of PSON.  Since we've now officially
adopted a dependency on the JSON gem everywhere, our code should run
free and flourish in the new world, where JSON processing is handled
through a less ridiculously named constant – this change fully realizes
that new world for the PMT.