NIFIREG-364 - Upgraded again to checkstyle 8.31.

Signed-off-by: Arpad Boda <aboda@apache.org>

This closes #270
diff --git a/pom.xml b/pom.xml
index 3091a39..ad06fe7 100644
--- a/pom.xml
+++ b/pom.xml
@@ -483,7 +483,7 @@
                         <dependency>
                             <groupId>com.puppycrawl.tools</groupId>
                             <artifactId>checkstyle</artifactId>
-                            <version>8.29</version>
+                            <version>8.31</version>
                         </dependency>
                     </dependencies>
                 </plugin>