[bootstrap.py] Instruct curl to follow redirect

Some RUSTUP_DIST_SERVER perform redirection when downloading
stage0 compiler. Curl should be able to follow that.

src/bootstrap/bootstrap.py

@@ -122,6 +122,7 @@ def _download(path, url, probably_big, verbose, exception, help_on_error=None):
             option = "-s"
         require(["curl", "--version"])
         run(["curl", option,
+             "-L", # Follow redirect.
              "-y", "30", "-Y", "10",    # timeout if speed is < 10 bytes/sec for > 30 seconds
              "--connect-timeout", "30",  # timeout if cannot connect within 30 seconds
              "--retry", "3", "-Sf", "-o", path, url],