Liquibase Gradle plugin appears to have url and referenceUrl reversed

Question

I have a very simple Spring Boot 2.0.4 project. Following the various examples for setting up the Liquibase Gradle plugin I wanted to be able to run the diffChangeLog target to update my change log XML file. The configuration looks like this:

liquibase {
    activities {
        main {
            url                 'jdbc:postgresql://localhost:5432/example_db'
            username            'user'
            password            'password'
            driver              'org.postgresql.Driver'

            referenceUrl        'hibernate:spring:com.example?dialect=org.hibernate.dialect.PostgreSQL9Dialect&hibernate.physical_naming_strategy=org.springframework.boot.orm.jpa.hibernate.SpringPhysicalNamingStrategy&hibernate.implicit_naming_strategy=org.springframework.boot.orm.jpa.hibernate.SpringImplicitNamingStrategy'
            referenceDriver     'liquibase.ext.hibernate.database.connection.HibernateDriver'

            classpath           'src/main'
            changeLogFile       "src/main/resources/db/changelog/master.xml"
        }
        runList = 'main'
    }
}

The database is empty except for the databasechangelog and databasechangeloglock tables. When I run gradle diffChangeLog the change log XML file is never updated. The output from gradle diff shows "NONE" for everything. My project does have an entity and it is annotated with @Entity.

What am I doing wrong?