Update main source and target JDK compatibility

Update source and target compatibility from 1.5 to 1.6, meaning that
@Override is now allowed on implementations of interface methods within
main classes.

Issue: SPR-10130

build.gradle

@@ -33,8 +33,8 @@ configure(allprojects) {
 	group = "org.springframework"
 
 	compileJava {
-		sourceCompatibility=1.5
-		targetCompatibility=1.5
+		sourceCompatibility=1.6
+		targetCompatibility=1.6
 	}
 	compileTestJava {
 		sourceCompatibility=1.7