Update install.php

"Pretty printing" `config.yml` in order to make it editable by humans. `5` is just value which is looking good for me. It produces a `config.yml` like this:

    prevstage:
        - start
        - database
        - github
        - email
    b8:
        database:
            servers:
                read:
                    - localhost
                write:
                    - localhost
            name: phpci
            username: *****
            password: *****
    phpci:
        url: 'http://phpci.my.org'
        github:
            id: ''
            secret: ''
        email_settings:
            smtp_address: ''
            smtp_port: ''
            smtp_encryption: on
            smtp_username: ''
            smtp_password: ''
            from_address: ''
            default_mailto_address: ''

I guess the `prevstage` node can be replaced before writing? If yes I'll prepare that.