Configured Java 1.6 as source and target compatibility

This patch raises the source and target compatibility to Java 1.6 as project already uses constructs available only since Java 1.6 (e.g. name method on StringRelationshipType, which is implementation of a method defined on RelationshipType interface, is annotated with @Override, while @Override can be placed on methods inherited from an interface only since Java 1.6).