Doesn't work with path fallbacks

[pawelgur]

When using path callbacks as array of paths http://requirejs.org/docs/api.html#pathsfallbacks, require.replace breaks.

This would be nice to have, because if for some reasone replaced download fails (which is possible with dynamic path), you could always specify a fallback.

"require.replace": "~0.3.0" "requirejs": "2.1"

[SBoudrias]

Pull request welcomed