Better testing: also run the test suite of one or more dependant projects

[marten-de-vries]

Currently most of the exposed dependencies aren't actually tested for changes. (There's no check if they adhere to semver, and even if they do, a major change in a dependency might still not be relevant to the usage of pouchdb-plugin-helper and could then be published in a patch release).

To improve this, it would be nice if Travis ran the test suite and build process of one or more dependants. E.g. pouchdb-auth would be a good candidate.