Upping source compatibility to 1.7. Even 1.7 is end-of-lifed by now

but at least it's newer than the ancient 1.6.

common.gradle

@@ -8,7 +8,7 @@ apply plugin: 'maven'
 group = 'org.jmonkeyengine'
 version = jmePomVersion
 
-sourceCompatibility = '1.6'
+sourceCompatibility = '1.7'
 [compileJava, compileTestJava]*.options*.encoding = 'UTF-8'
 
 repositories {