Bump plv8 version from 2.3.13 to 2.3.15

[GradedJestRisk]

Issue #36
Local build ended successfully => have a try on my DockerHub

docker run --detach --env POSTGRES_HOST_AUTH_METHOD=trust --publish 5432:5432 --name postgres_node gradedjestrisk/plv8:12-2
CREATE EXTENSION plv8;
SELECT extversion FROM pg_extension WHERE  AND extname = 'plv8';
-- 2.3.15

Look like the CI was already failing before this PR, shall we block the merge and fix it beforehand ?
#37 (comment)