Upgrade to multer 1.4.0

[jonaskello]

New version of multer is available, could this package be upgraded?

[shanvl]

I've made a pull request

[niftylettuce]

Please see my comment at #30 (comment) and close this issue. The official Koa organization has forked this repository and released it (with PR's merged) under @koajs/multer. Thank you.