module: allow require('.')

Previously, the minimal argument to require the current directory was
require('./'). This commits allows to skip the trailing slash.

Fixes: https://github.com/iojs/io.js/issues/1178
PR-URL: https://github.com/iojs/io.js/pull/1185


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Christian Tellnes <christian@tellnes.no>
Reviewed-By: Roman Reiss <me@silverwind.io>