boost 1.7+ breaks compilation

[XapaJIaMnu]

Hey,

Compilation is broken with boost version>= 1.7 (released in April 2019) due to change in the interface:

/mnt/Storage/mts/3rd_party/crow/include/crow/socket_adaptors.h: In member function ‘boost::asio::io_service& crow::SocketAdaptor::get_io_service()’:
/mnt/Storage/mts/3rd_party/crow/include/crow/socket_adaptors.h:22:28: error: ‘boost::asio::ip::tcp::socket’ {aka ‘class boost::asio::basic_stream_socket<boost::asio::ip::tcp>’} has no member named ‘get_io_service’
   22 |             return socket_.get_io_service();
      |                            ^~~~~~~~~~~~~~

More info here:
rstudio/rstudio#4636

We should at least have that in the requirements.

Cheers,

Nick