After multiple users filing issues about material-ui-icons it was found that material-ui has moved the icon package to a new location. It is now under the material-ui namespace as opposed to being on its own under material-ui-icons. Because of this, I've decided to increase the major version. Also keeping in mind that Material-UI will soon be releasing V1 (May 17th) I've moved this to a beta so that in the mean time users experiencing issues can at least get back up and working again.
Thanks to @baer for the PR which resolved the issues