src: fix deprecated use of Buffer::New()

Pass the isolate explicitly.  Overlooked in commit ccb199af ("src: fix
deprecation warnings") because g++ 4.8 and 4.9 don't warn for it
whereas g++ 5.1 does.

PR-URL: https://github.com/iojs/io.js/pull/1577


Reviewed-By: Trevor Norris <trev.norris@gmail.com>