MongoDB Upgrade Error
If you should ever encounter an error message along the lines of the following, go to the StackOverflow link lower down and follow the instructions carefully - it works!
My best guess is that the problem was caused by running $ brew update followed by $ brew upgrade giving a version conflict somewhere along the line.
** IMPORTANT: UPGRADE PROBLEM: The data files need to be fully upgraded to version 3.6 before attempting an upgrade to 4.0; see [http://dochub.mongodb.org/core/4.0-upgrade-fcv](http://dochub.mongodb.org/core/4.0-upgrade-fcv) for more details.