Update p5-Sereal and friends to 3.015.

While here, a number of Makefile tweaks, including a little
more info in the descrs, meta::cpan instead of cpan for WWW,
and a number of formatting tweaks.

Also, given that p5-Sereal, -Decoder, and -Encoder all have
the same tests, it's really no benefit to making -Decoder
and -Encoder register TEST_DEPENDS on each other. Tests for
just that module work for 'make test', and the full suite
can be run by doing 'make test' in the main p5-Sereal port.

Also, take maintainership. This is the third PR that's timed
out, and I have sent multiple direct emails with each one.
kuriyama's last commit to this port was nearly 2 years ago.
I am considering this port abandoned. I'd like to thank
kuriyama for creating these ports and looking after them
for over 3 and a half years!

PR: 212316
Approved by: maintainer timeout