Is there any specific reason or issue preventing official support for PostgreSQL 9.4.x? Or it is just not tested yet?
Looking at the Linux world, PostgreSQL is becoming quite common
Debian oldstable / wheezy has 9.1 and no plv8 package
Debian stable / jessie has 9.4 and it does have the plv8 package for 9.4 too:
Ubuntu's current stable release, 15.04 (Vivid Vervet) and the next stable release, 15.10 (Wily Werewolf) has 9.4:
Ubuntu LTS 14.04 (Trusty Tahr) has 9.3:
RHEL 7 / CentOS 7 have PostgreSQL 9.2
Fedora 22 and 23 have PostgreSQL 9.4.x