Fixed dependency on libssh2 in README
Fixed dependency on libssh2 in README

As I don't know the correct version of libssh is checked (I supposed there is no such check), the version of libssh on which it depends has changed following ba721ccbedc743a29efe3a4921999bdc3f016ff9 (introduces libssh2_session_handshake which does not exists pre libssh2-1.2.8 http://www.libssh2.org/changes.html).
wallix/pylibssh2,wallix/pylibssh2,wallix/pylibssh2