Resolve unchecked type casting in Chunk and ChunkRepository

[notjuanortiz]

> Task :compileJava
/home/travis/build/luna-rs/luna/src/main/java/io/luna/game/model/chunk/Chunk.java:83: warning: [unchecked] unchecked cast
        return (Stream<E>) getAll(type).stream();
                                              ^
  required: Stream<E>
  found:    Stream<Entity>
  where E is a type-variable:
    E extends Entity declared in method <E>stream(EntityType)
/home/travis/build/luna-rs/luna/src/main/java/io/luna/game/model/chunk/Chunk.java:94: warning: [unchecked] unchecked cast
        return (Iterator<E>) getAll(type).iterator();
                                                  ^
  required: Iterator<E>
  found:    Iterator<Entity>
  where E is a type-variable:
    E extends Entity declared in method <E>iterator(EntityType)
/home/travis/build/luna-rs/luna/src/main/java/io/luna/game/model/chunk/ChunkRepository.java:70: warning: [unchecked] unchecked cast
        return (Set<E>) entities.get(type);