Remove support for postgres 9.4

[rjuju]

This version has been EOL for a very long time, and supporting it just became too expensive. The main reason to drop this versions are:

• pg_event_trigger_ddl_commands() is not there, making it impossible to detect what extension has just been created or dropped in an event trigger. Previous versions of PoWA were blindly trying to register or unregister all supported extensions when any extension is created or dropped, but that's not a sensible approach and PoWA 5 checks for the actual extension
• Using named operators in function calls only works with the old (and not standard) syntax ":=" rather than "=>". Having to stick with the old syntax is eventually going to byte us
• ASSERT is not supported in plpgsql. While not critical it's nice to have and adding some client-side layer doesn't make sense