Made the map implementations Java 8 compliant.

Using a JRE or JDK 8, the libgdx map implementations actually have a compile error. As an example:

> Duplicate methods named remove with the parameters () and () are inherited from the types `ObjectMap.MapIterator<K,V>` and `Iterator<ObjectMap.Entry<K,V>>`

This is the declaration of the class with the error:

    static public class Entries<K, V> extends MapIterator<K, V> implements Iterable<Entry<K, V>>, Iterator<Entry<K, V>> {

That's because with Java 8 `Iterator` now also provides a default implementation for `remove()`. Since `MapIterator` also does that, there's a conflict. I resolved it by explicitly overriding the method in all classes which have this conflict and solve it by saying which one should be used, without using the Java 8 syntax to do so. That way it should still be compliant with Java 6 compilers, while at the same time supporting Java 8 compilers.