Coax EUMM into producing META 2.0 META.json

As per #59.

The good folks in #toolchain knew a trick that would make it work.

However, its a tentative pull req, because I don't know what all the right deps are for different phases.

It appears to do the right thing, however:
 
- It required writing the resources entry in META 2.0 Format: https://metacpan.org/pod/CPAN::Meta::Spec#resources
- I moved only deps I could find in t/ to `TEST_REQUIRES`
- There's a bunch of deps that are declared, but I can't see them used anywhere by a source grep, and so I left them in place.
- I'm not entirely sure why `Sereal` declares `Sereal::Encode`  and `::Decode` as both `BUILD_` and `PREREQ_PM`, it might have just been a phasing thing or and accident or ????

=~ I totally anticipate that this pull request won't be merged as-is, but I'll adjust it as needs require if you want, or if you wish to steal parts from it and not merge it, that's cool too =)