Fixed an issue with an old version of libxmljs dependency

Installing the excel parser on a newer version of node and OS X resulted in an error in libxmljs. See also issue 141 at libxmljs: https://github.com/polotek/libxmljs/issues/141

I forked from @mako-taco's fork which already included node-style err in the callback: `function(err, data) {}`.