[Low priority] Tweak dev setup to clone via https

Before, trying to `git clone git@github.com:exercism/cli.git` would
result in an error because non-repo-collaborators don't have git-ssh
access.

    > git clone git@github.com:exercism/cli.git src/github.com/exercism/cli
    Cloning into 'src/github.com/exercism/cli'...
    Permission denied (publickey).
    fatal: Could not read from remote repository.

    Please make sure you have the correct access rights
    and the repository exists.

This is a *very* minor change, but maybe one that will make it
marginally easier to jump into development of exercism/cli.