switch to CouchDB 3 for the Travis tests - no admin party in CouchDB 3

[glynnbird]

Overview

CouchDB 3 is released 💯 , so the Travis tests need to use that as a target. This happens automatically, because travis pulls couchdb:latest, but CouchDB 3 has no "admin party", so we need to create an admin user otherwise nothing works.

Testing recommendations

export TRAVIS=true
npm run test

Checklist

• [x] Code is written and works correctly;
• [x] Changes are covered by tests;
• [x] Documentation reflects the changes;