Upgrade PMD to version 6.0.1 (#23)

* Upgrade PMD to version 6

* Adjust regression suite as the new version has different output values

* Use new rules xml instead of group name, as they became incompatible

* Speedup analysis with builtin cache

* Redirect warnings to stderr

* Hardcode ruleset as runtime directory changes

Author: filipesperandio

Diff for: bin/install-pmd.sh

@@ -4,7 +4,7 @@ set -euo pipefail
 LIB_DIR=/usr/src/app/lib
 
 download_pmd() {
-  URL="https://github.com/pmd/pmd/releases/download/pmd_releases/5.8.1/pmd-bin-5.8.1.zip"
+  URL="https://github.com/pmd/pmd/releases/download/pmd_releases/6.0.1/pmd-bin-6.0.1.zip"
   wget -O pmd.zip $URL
 }