Heroku and plv8 support error

Question

I would like to use the plv8 extension on heroku. I followed the official tutorial, but however when I connect to psql console on heroku and try to add the extension:

create extension PLV8;

I get the following error:

ERROR:  could not open extension control file "/usr/share/postgresql/9.5/extension/plv8.control": No such file or directory

I checked the location by connecting via bash to my heroku app and this file is not available. I also cannot create it since it is a read-only filesystem.

I have bought the standard-0 plan which is prerequisite to use plv8. When I enter:

echo 'show extwlist.extensions' | heroku pg:psql

I get:

extwlist.extensions                                                        
 btree_gin,btree_gist,chkpass,citext,cube,dblink,dict_int,earthdistance,fuzzystrmatch,hstore,intarray,isn,ltree,pg_partman,pg_prewarm,pg_stat_statements,pg_trgm,pgcrypto,pgrowlocks,pgstattuple,plpgsql,plv8,postgis,postgis_topology,postgres_fdw,redis_fdw,tablefunc,unaccent,uuid-ossp,xml2
(1 row)

which shows that plv8 is available as extension.

The question now is if anyone has successfully enabled plv8 on heroku yet and how?

Thanks in advance for all your help.