Handle changes to boost::optional in newly released boost 1.56:

To improve compatibility with the proposed std::optional a number
of changes were made, one of which is the removal of the implicit
conversion to bool.  As a result, returning boost::optional as a
bool value now fails.  Explicit conversion to bool used for clarity.

src/ripple/module/app/tx/TransactionMeta.h

@@ -107,7 +107,7 @@ public:
 
     bool hasDeliveredAmount () const
     {
-         return mDelivered;
+        return bool(mDelivered);
     }
 
     static bool thread (STObject& node, uint256 const& prevTxID, std::uint32_t prevLgrID);