Change protocol from git:// to https:// when cloning from GitHub

[kmaehashi]

Under the network environment that require proxies (e.g., enterprise network), HTTPS is much preferable to Git protocol, as we can just use https_proxy environment variable instead of setting up custom Git proxy command.

[odasatoshi]

I had misunderstood that hg protocol doesn't through http proxys. I merged your commit. thanks.