update gossh to latest go.crypto/ssh changes

go.crypto/ssh changed its API (see [1]). There is one issue (see [2]) that
required the ugly "else if" construction.

[1] https://groups.google.com/forum/#!topic/golang-nuts/AoVxQ4bB5XQ
[2] https://code.google.com/p/go/issues/detail?id=7787