Upgrade pmd to 5.6.1 to avoid NPE (#426)
Upgrade pmd to 5.6.1 to avoid NPE (#426)

With the pmd version 5.5.4 we're now getting:

```
Exception applying rule InvalidSlf4jMessageFormat on file servo/servo-core/src/main/java/com/netflix/servo/DefaultMonitorRegistry.java, continuing with next rule
java.lang.NullPointerException
```
brharrington/servo,Netflix/servo,brharrington/servo