Replace `{dir}` in gemsrc_clone_root with the repository url.

For instance, when the repository is `https://github.com/foo/bar` and `GEMSRC_CLONE_ROOT` is set to `/path/to/src/{dir}`, the destination directory will be `/path/to/src/github.com/foo/bar`.

If all gems are cloned to /path/to/src, there will be too many directory to find the one I'd like to cd.

The directory structure will be like the following:

```
$ tree -L 3
.
├── github.com
│   ├── amatsuda
│   │   ├── active_decorator
│   │   └── i18n_generators
│   ├── jimweirich
│   │   ├── builder
│   │   └── rake
│   ├── jnicklas
│   │   ├── capybara
│   │   └── xpath
│   ├── jonleighton
│   │   └── poltergeist
│   ├── lostisland
│   │   └── faraday
│   ├── net-ssh
│   │   ├── net-ssh
│   │   └── net-ssh-gateway
│   ├── rack
│   │   └── rack
│   ├── rails
│   │   ├── arel
│   │   ├── rails
│   │   └── sprockets-rails
│   ├── rspec
│   │   ├── rspec
│   │   ├── rspec-core
│   │   ├── rspec-expectations
│   │   └── rspec-mocks
```