[SourcesManager] Small cleanup
[SourceManager] Disable remote check on update in favour of a warning

Closes https://github.com/CocoaPods/CocoaPods/issues/2388
[SourcesManager] Removed unused #git_remote_reachable? method
[Changelog] Credit is due where credit is deserved
[Gemspec] Remove post install message
[Changelog] irrationalfab is now FabioPelosin