Fix http client on boost <= 1.46

[mpapierski]

https://travis-ci.org/mpapierski/asio_http/builds/29204548

Turns out boost::asio::async_connect appears in boost > 1.46 versions. It is good idea to support C++03 and older Boost versions at this point.

boost::asio::async_connect is used to do asynchronous connect to a series of endpoints. We can simulate this using socket::async_connect and incrementing endpoint iterator returned from async_resolve.