Fix. scope._cb is object not a 'function' error

TypeError: Property '_cb' of object [object Object] is not a function
    at null.<anonymous> (/home/lmj/project/huaban/readability/node_modules/minreq/index.js:119:10)
    at EventEmitter.emit (events.js:117:20)
    at IncomingMessage.<anonymous> (/home/lmj/project/huaban/readability/node_modules/minreq/index.js:194:28)
    at IncomingMessage.EventEmitter.emit (events.js:117:20)
    at _stream_readable.js:870:14
    at process._tickCallback (node.js:415:13)