pouchdb-migrate has an unnecessary dependency on pouchdb

[lbeschastny]

pouchdb-migrate should not depend on pouchdb itself. Especially since pouchdb is just on of many PouchDB builds obtained from pouchdb-core package and some plugins.

Right now pouchdb-migrate depend on pouchdb package in two ways.

First, it uses 'pouchdb/extras/promise', which was moves into a separate package pouchdb-promise. This package should be used instead.

Second, it's used in tests. But for this purpose pouchdb could be moved to devDependencies section.