next: Upgrade V8 to 4.3.61.19

• @indutny's SealHandleScope patch has been cherry picked onto the top of V8 to make it compile.
• There's some test breakage in contextify/vm.

I'll need some help sorting out the vm failures:

$ ./iojs test/parallel/test-vm-new-script-new-context.js 
run a string
script created
thrown error
undefined reference
evalmachine.<anonymous>:1
foo.bar = 5;
        ^
TypeError: Cannot set property 'bar' of undefined
    at evalmachine.<anonymous>:1:9
    at ContextifyScript.Script.runInNewContext (vm.js:18:15)
    at /Users/chris/projects/iojs/test/parallel/test-vm-new-script-new-context.js:27:10
    at Function._throws (assert.js:292:5)
    at Function.assert.throws (assert.js:318:11)
    at Object.<anonymous> (/Users/chris/projects/iojs/test/parallel/test-vm-new-script-new-context.js:26:8)
    at Module._compile (module.js:431:26)
    at Object.Module._extensions..js (module.js:449:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:311:12)

And

$ ./iojs test/message/undefined_reference_in_new_context.js
before
evalmachine.<anonymous>:1
foo.bar = 5;
        ^
TypeError: Cannot set property 'bar' of undefined
    at evalmachine.<anonymous>:1:9
    at ContextifyScript.Script.runInNewContext (vm.js:18:15)
    at Object.exports.runInNewContext (vm.js:49:17)
    at Object.<anonymous> (/Users/chris/projects/iojs/test/message/undefined_reference_in_new_context.js:8:4)
    at Module._compile (module.js:431:26)
    at Object.Module._extensions..js (module.js:449:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:311:12)
    at Function.Module.runMain (module.js:472:10)
    at startup (node.js:124:18)

R=@domenic, @indutny, @bnoordhuis