V8 version is set by the topmost script

Change-Id: I86a3796168b7064986a43c6db721477bd0fc6ad3
Reviewed-on: http://review.couchbase.org/32864
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>