repl: accept no arguments to start()

Currently, there is a check to ensure that the user either
provides an object or a string to repl.start(). The string case
is used to set a REPL prompt. However, a default of '> ' already
exists, so forcing the user to specify a prompt is a bit
redundant. This commit removes this restriction.

Fixes: https://github.com/nodejs/node/issues/5385
PR-URL: https://github.com/nodejs/node/pull/5388


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Julian Duque <julianduquej@gmail.com>