Switch default repository GitHub URLs to https:// instead of git://

Running ``npm init`` in an GitHub repo with an SSH URL (eg ``git@github.com:user/repo.git``) defaults to using ``git://`` as its repository scheme in the resulting ``package.json``.  

However, GitHub recommends & promotes using ``https://`` instead of ``git://``.  From a support email written by Kyle, a GitHub employee:

>  There are several problems with the git:// protocol (completely unencrypted and open to man in the middle attacks for instance). HTTPS should be beneficial in every way for your uses (it's actually just as efficient as git:// since the SmartHTTP update 3 years ago). For the security reasons, we've decided to stop promoting the protocol. That being said, we will still support it — but you'll have to manually type in the URLs.

Side note: I did ask them to add the above to their [documentation](https://help.github.com/articles/which-remote-url-should-i-use) after they told me this, but no such luck.  I suppose it's effectively implied, though not very clearly.