deps: upgrade v8 to 4.1.0.21
R=@indutny, /cc @iojs/tc
This upgrade is an interesting conundrum. There is an incompatible ABI (but not API) change in here that would require bumping NODE_MODULE_VERSION and NODE_MINOR_VERSION.
However, skipping the upgrade is probably a bad idea because it also contains a fix for https://code.google.com/p/chromium/issues/detail?id=430201, which annoyingly is still under embargo but looks like it's a use-after-free and may be exploitable.
As a proposed fix, I backed out the ABI change. WDYT?