debugger: use requireRepl() to load debugger repl

Currently, the debugger uses require('repl') to setup the repl.
However, require.extensions is not available yet, causing a
crash on tab completion of require('. This commit uses the
module.requireRepl() method to bootstrap the repl.

Fixes: https://github.com/joyent/node/issues/8359
PR-URL: https://github.com/iojs/io.js/pull/49
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>