Fixes #720 replace lambda with method ref

exercises/allergies/src/example/java/Allergies.java

 
     public List<Allergen> getList() {
         return EnumSet.allOf(Allergen.class).stream()
-                .filter(x -> isAllergicTo(x))
+                .filter(this::isAllergicTo)
                 .collect(Collectors.toList());
     }