change default git_url from "git://" to "https://"

For the following reasons, the default git_url should be "https://".
- More workable-everywhere even if behind a firewall or proxy.
- You can go around the following workaround:
  "git config --global url."https://".insteadOf git://"
  However, we do care about "chef" config, not "git" config,
  especially using Berkshelf.
- github's default URL for cloning seems "https://" now.