Fix reference types in iterators

Amends 26faac38.

include/yaml-cpp/node/detail/iterator.h

@@ -41,7 +41,7 @@ class iterator_base {
   using value_type = V;
   using difference_type = std::ptrdiff_t;
   using pointer = V*;
-  using reference = V;
+  using reference = V&;
 
  public:
   iterator_base() : m_iterator(), m_pMemory() {}

include/yaml-cpp/node/detail/node_iterator.h

@@ -69,7 +69,7 @@ class node_iterator_base {
   using value_type = node_iterator_value<V>;
   using difference_type = std::ptrdiff_t;
   using pointer = node_iterator_value<V>*;
-  using reference = node_iterator_value<V>;
+  using reference = node_iterator_value<V>&;
   using SeqIter = typename node_iterator_type<V>::seq;
   using MapIter = typename node_iterator_type<V>::map;