jme3-plugins: upgradel the "gson" library to v2.8.9

jme3-plugins/build.gradle

@@ -11,6 +11,6 @@ sourceSets {
 
 dependencies {
     api project(':jme3-core')
-    api 'com.google.code.gson:gson:2.8.1'
+    api 'com.google.code.gson:gson:2.8.9'
     testImplementation project(':jme3-desktop')
 }