refactor: use core/ref.hpp over ref.hpp

The later has been deprecated:
```cpp

// The header file at this path is deprecated;
// use boost/core/ref.hpp instead.

```

example/cookbook/do_the_bind.cpp

@@ -46,7 +46,7 @@
 #include <boost/mpl/max.hpp>
 #include <boost/mpl/next.hpp>
 
-#include <boost/ref.hpp>
+#include <boost/core/ref.hpp>
 #include <iostream>
 #include <typeinfo>
 

include/boost/fusion/support/deduce.hpp

@@ -10,7 +10,7 @@
 #define BOOST_FUSION_SUPPORT_DEDUCE_HPP_INCLUDED
 
 #include <boost/fusion/support/config.hpp>
-#include <boost/ref.hpp>
+#include <boost/core/ref.hpp>
 
 #ifndef BOOST_NO_CXX11_HDR_FUNCTIONAL
 #include <functional>

include/boost/fusion/support/detail/as_fusion_element.hpp

@@ -9,7 +9,7 @@
 #define FUSION_AS_FUSION_ELEMENT_05052005_0338
 
 #include <boost/fusion/support/config.hpp>
-#include <boost/ref.hpp>
+#include <boost/core/ref.hpp>
 
 #ifndef BOOST_NO_CXX11_HDR_FUNCTIONAL
 #include <functional>

test/functional/make_unfused.cpp

@@ -19,11 +19,11 @@
 
 #include <boost/utility/result_of.hpp>
 #include <boost/core/enable_if.hpp>
+#include <boost/core/ref.hpp>
 
 #include <boost/fusion/sequence/intrinsic/empty.hpp>
 #include <boost/fusion/algorithm/iteration/fold.hpp>
 
-#include <boost/ref.hpp>
 
 namespace fusion = boost::fusion;
 namespace mpl = boost::mpl;

test/sequence/deduce_sequence.cpp

@@ -13,7 +13,7 @@
 
 #include <boost/mpl/equal.hpp>
 
-#include <boost/ref.hpp>
+#include <boost/core/ref.hpp>
 #ifndef BOOST_NO_CXX11_HDR_FUNCTIONAL
 #include <functional>
 #endif