Remove 'ssh://' from github ssh_url

git-clone does not work if 'ssh://' is prepended:

    $ git clone ssh://git@github.com:libwww-perl/libwww-perl.git
    Cloning into libwww-perl...
    ssh: Could not resolve hostname github.com:libwww-perl: Name or service not known
    fatal: The remote end hung up unexpectedly

Just using the ssh_url as it comes from api.github.com works:

    $ git clone git@github.com:libwww-perl/libwww-perl.git
    Cloning into libwww-perl...
    (works)
Merge pull request #1017 from eserte/github-ssh-url

Remove 'ssh://' from github ssh_url