add a peer dependency on mongoose to satisfy pnpm's strict dependency…

[restjohn]

This pull request simply adds Mongoose @ versions 4-5 to to the peer dependencies of the sinon-mongoose package. This satisfies the strictness constraints of dependency managers pnpm and Yarn 2.

[restjohn]

Unfortunately my pnpm build would not work when I added a restjohn/sinon-mongoose#pnpm git dependency because pnpm does not pull the dev dependencies and run the prepare script of git dependencies (yet). However, I still believe the change to the prepare script is valid because it works for npm and Yarn. The only slight issue is that npm also runs the prepare script when running npm install within a cloned sinon-mongoose working tree, the reason for which I'm not quite sure. Yarn behaves similarly.