Avoid upgrade issue

Port plv8/plv8#552 on version 3.1.x

Upgrade process between 3.1.4 and 3.1.5 fails with a segfault when
running ALTER EXTENSION plv8 UPDATE TO '3.1.5'.

To avoid relying on the upgrade code, we replace the upgrade process by
dropping the extension first and then install the new version.

Author: jfroche