Revert to v8 3.9.7

Change-Id: I09bbe5d6e184629b13b55afffb74f47e8711a3b8
Reviewed-on: http://review.couchbase.org/34722
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>