ebuild: depend on <dev-lang/v8-3.16.5:= for 0.8 ebuilds

I forgot to cherry-pick 304b183fd53e9851c21d1c61ed53b7fe551abf1b
Merge pull request #502 from karolherbst/ebuild

ebuild: depend on <dev-lang/v8-3.16.5:= for 0.8 ebuilds