Rename META.info -> META6.json

Andreas Koenig pointed out that .info is being used for quite some other things
on the Open Source world.  So we (FROGGS + lizmat) decided to change the name
in such a way, that it is more in line with the world and still an easy way to
distinguish a Perl 6 distribution from other distributions.
Merge branch 'master' of https://github.com/perl6/specs