Travis CI builds don't run documentation tests

[cmbrandenburg]

Currently, the Travis CI builds don't run documentation tests. This is because Cargo runs documentation tests only if the command line specifies no test filter. However, we use the test filter --lib to avoid running any integration tests, which would fail because the Travis CI build agent doesn't have a CouchDB server installed.

Therefore, resolving this issue will require better support for running documentation tests using Cargo.

[jgillich]

You should be able to get a CouchDB by setting services: couchdb. See https://docs.travis-ci.com/user/database-setup/#CouchDB

[cmbrandenburg]

@jgillich, this is good to know, thanks!