I encountered unexplainable issues (about liquibase dependancies, classpath, jar inclusion) using liquibase command line interface in my macosX terminal.
I eneded up using the intellij to run liquibase successfully.
Hereunder the command line to generate a YML changelog of an existing database running on localhost.
--driver=org.postgresql.Driver
--classpath=/Users/Xavier/.m2/repository/org/postgresql/postgresql/9.4.1212.jre7/postgresql-9.4.1212.jre7.jar
--changeLogFile=db.changelog.yml