Upgrade maven-pmd-plugin to latest version

Change-Id: Ief52488d97bb2771b4c5a40c4a9578c68a24964b
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

pom.xml

diff --git a/pom.xml b/pom.xml
index fb94dfe..bcd2428 100644
--- a/pom.xml
+++ b/pom.xml
@@ -390,7 +390,7 @@
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-pmd-plugin</artifactId>
-          <version>3.7</version>
+          <version>3.8</version>
           <configuration>
             <sourceEncoding>utf-8</sourceEncoding>
             <minimumTokens>100</minimumTokens>