Changed git download protocol from git:// to https:// to work around firewalls.

I was trying to write an automated script for using nvm in the development toolchain at work. However, the `install.sh` script would fail to complete with a timeout error. I discovered that the script used `git clone git://github.com/creationix/nvm.git $NVM_TARGET` to download the repo. I changed the protocol from `git://` to `https://` and everything works as I expected. I don't believe there is a downside to this change.

Thank you for the great project, and thank you for considering my pull request.

Chip Warden