www/rubygem-passenger: unbreak with libc++ 7

In file included from src/agent/Watchdog/WatchdogMain.cpp:68:
In file included from src/agent/Watchdog/Config.h:32:
In file included from src/agent/Core/Config.h:47:
In file included from src/agent/Core/AdminPanelConnector.h:43:
In file included from src/cxx_supportlib/WebSocketCommandReverseServer.h:29:
In file included from src/cxx_supportlib/vendor-copy/websocketpp/websocketpp/config/asio_no_tls_client.hpp:32:
In file included from src/cxx_supportlib/vendor-copy/websocketpp/websocketpp/transport/asio/endpoint.hpp:32:
In file included from src/cxx_supportlib/vendor-copy/websocketpp/websocketpp/transport/asio/connection.hpp:31:
In file included from src/cxx_supportlib/vendor-copy/websocketpp/websocketpp/transport/asio/base.hpp:31:
In file included from src/cxx_supportlib/vendor-copy/websocketpp/websocketpp/common/asio.hpp:65:
In file included from src/cxx_supportlib/vendor-modified/boost/asio.hpp:23:
In file included from src/cxx_supportlib/vendor-modified/boost/asio/basic_datagram_socket.hpp:20:
In file included from src/cxx_supportlib/vendor-modified/boost/asio/basic_socket.hpp:40:
In file included from src/cxx_supportlib/vendor-modified/boost/asio/detail/reactive_socket_service.hpp:22:
In file included from src/cxx_supportlib/vendor-modified/boost/asio/buffer.hpp:27:
In file included from src/cxx_supportlib/vendor-modified/boost/asio/detail/string_view.hpp:25:
/usr/include/c++/v1/experimental/string_view:11:2: error: "<experimental/string_view> has been removed. Use <string_view> instead."

PR: 230401
Reported by: antoine (via exp-run)