databases/postgresql-plruby: Unbreak build with PostgreSQL 11

PostgreSQL 11.4

is unsupported. Try plruby-0.4.2.

• Error code 1

http://package18.nyi.freebsd.org/data/120amd64-default-PR239514/2019-07-29_20h41m52s/logs/postgresql-plruby-0.5.7_4.log

While here deprecate it since it seems to have been abandoned in

PR: 239639
Approved by: knu (maintainer timeout, 2 weeks)