Connecting to github broken due to outdated libssh2 #644

Github has deprecated a number of crypto protocols that are used in the 5 year old version of libssh2 (@ficfa55) referenced by objective-git. See https://githubengineering.com/crypto-deprecation-notice/

This is causing Unable to exchange encryption keys errors preventing the ssh session from being established. This is likely the root cause of the issue for #641 as well. (In fact, I believe he may be experiencing problems with the same app that I am, passforios https://github.com/mssun/passforios.

I don't program for iOS so I don't have the toolchain to provides a pull request.