I just did an install of NodeBB 0.8.2 and it installed nodebb-plugin-emailer-mandrill 0.3.1 with it which is only compatible with NodeBB 0.9.^. In addition to the declared version problem I was also getting a socket.io/helpers related error on NodeBB startup until I removed the mandrill plugin.
I worked around this problem by removing mandrill and then installing it like this:
npm install firstname.lastname@example.org
I poked around to see how nodebb installs nodebb-plugin-emailer-mandrill and it appears that this plugin is installed in a special way? It is not mentioned in the package.json requirements for nodebb.