Fix TLS errors when downloading stage0

Author: dlrobertson

src/bootstrap/bootstrap.py

@@ -79,8 +79,8 @@ def _download(path, url, probably_big, verbose, exception):
     # see http://serverfault.com/questions/301128/how-to-download
     if sys.platform == 'win32':
         run(["PowerShell.exe", "/nologo", "-Command",
-             "(New-Object System.Net.WebClient)"
-             ".DownloadFile('{}', '{}')".format(url, path)],
+             "[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12;",
+             "(New-Object System.Net.WebClient).DownloadFile('{}', '{}')".format(url, path)],
             verbose=verbose,
             exception=exception)
     else: