Revert Perl6::MIME::Base64 to working commit.

moritz had fixed this last month, I just didn't know about it when I
updated this module.
Update release guide to deal with Perl6::MIME::Base64

There's no easy way to modify the existing submodule command to skip
Perl6::MIME::Base64, in a cross-platform way. So instead presented here
is a way to fix Perl6::MIME::Base64 getting a merge commit.

The sooner we don't need this addition to the Release Guide, the better.