Remove leading white space in the config file.

`json.dumps` uses the seperator `', '` by default. So when indent mode is enabled extra white space is added after each comma. The [docs](http://docs.python.org/2/library/json.html) recommend using `separators=(',', ': ')` (Note under Basic Usage) to avoid this.

Before: (Extra whitespace after commas.   
 
    cat ~/.httpie/config.json -A
    {$
        "__meta__": {$
            "about": "HTTPie configuration file", $
            "help": "https://github.com/jkbr/httpie#config", $
            "httpie": "0.6.0"$
        }, $
        "default_options": [], $
        "implicit_content_type": "json"$
    }$

Fixed: 
 
    {$
        "__meta__": {$
            "about": "HTTPie configuration file",$
            "help": "https://github.com/jkbr/httpie#config",$
            "httpie": "0.6.0"$
        },$
        "default_options": [],$
        "implicit_content_type": "json"$
    }$