Fix ruby path in gitAndTools.hub

`hub` won't work on my NixOS system with the current master branch:

    $ hub
    /run/current-system/sw/bin/hub: /nix/store/j8fazj40azabllknnppbwv7yhpsfwpf6-hub-1.12.2/bin/.hub-wrapped: /usr/bin/ruby: bad interpreter: No such file or directory
    /run/current-system/sw/bin/hub: line 3: /nix/store/j8fazj40azabllknnppbwv7yhpsfwpf6-hub-1.12.2/bin/.hub-wrapped: Success

The problem is that `hub` searches in `lib/hub/standalone.rb` for `'/usr/bin/ruby'` and fails if that exact path can't be found.

/cc @the-kenny