Update to 1.975
    
    - New upstream release 1.975
      - BEHAVIOR CHANGE: work around TEA misfeature on OS X built-in openssl, e.g.
        guarantee that only the explicitly-given CA or the openssl default CA will
        be used; this means that certificates inside the OS X keyring will no
        longer be used, because there is no way to control the use by openssl
        (e.g. certificate pinning etc.)
      - Make external tests run by default to make sure default CA works on all
        platforms; it skips automatically on network problems like timeouts or SSL
        interception, and can also use http(s)_proxy environment variables