Update gson due to reported vulnerability.

Author: rpseng

OCPP-J/build.gradle

@@ -4,7 +4,7 @@
 
 dependencies {
     compile project(':common')
-    compile 'com.google.code.gson:gson:2.8.0'
+    compile 'com.google.code.gson:gson:2.8.9'
     compile 'org.java-websocket:Java-WebSocket:1.5.3'
     testCompile 'junit:junit:4.13.2'
     testCompile 'org.mockito:mockito-core:4.11.0'