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.

```

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