You didn;t put it in META-INF this time. You are getting there.. Slowly but surely.
Put it in src/main/resources/META-INF/persistence.xml. Also make sure that it shows up in the target/classes/META-INF after you run the build. Spring definetly needs the persistence.xml in META-INF folder in the classpath. So, it will never work unless it comes up in target/classes/META-INF
If that doesn;t work, you might have to turn springframework logging to DEBUG. I am pretty sure it logs where it's looking for persistence.xml