(pouchdb/pouchdb#2273) - get rid of peer dependency

I thought we already got rid of this.

Anyway, we're apparently using the peer dependency
incorrectly for mapreduce, because it breaks
projects that depend on pouchdb when they try
to do `npm shrinkwrap`.

Peer dependencies are designed for true plugins,
not dependencies of the main module. In our
terminology mapreduce is a "plugin," but for
the purposes of npm it's not.