libssh2: upgrade to v1.6.0

Besides new features and improvements, this fixes CVE-2015-1782.

License's md5 changed because of a modified copyright.

Signed-off-by: Hugo Vasconcelos Saldanha <hugo.saldanha@aker.com.br>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>