process: throw on non-function to nextTick()

This commit causes process.nextTick() to throw when its callback
argument is not a function.

PR-URL: https://github.com/nodejs/node/pull/3860


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>