Adjust new html format

ログアウト状態の場合のHTMLの構造が変更されていたので、対応しました。
よろしくお願いします。

```ruby
> Mechanize.new.get(GREE::Community.new(xxx).recent_threads_uri).forms
=> [#<Mechanize::Form
  {name nil}
  {method "POST"}
  {action "https://pid.gree.net/"}
  {fields
   [hidden:0x3fdcf0d9cabc type: hidden name: pid_csrf[token] value: yyy]
   [hidden:0x3fdcf0d9c954 type: hidden name: action value: login_web_commit]
   [hidden:0x3fdcf0d9c800 type: hidden name: backto value: http://gree.jp/?mode=community&act=bbs_list&community_id=xxx]
   [hidden:0x3fdcf0d9c698 type: hidden name: ithk value: ]
   [hidden:0x3fdcf0d9c544 type: hidden name: from_glogin value: login_login]
   [field:0x3fdcf0d9c3f0 type: email name: mail value: ]
   [field:0x3fdcf0d9c29c type: password name: user_password value: ]}
  {radiobuttons}
  {checkboxes}
  {file_uploads}
  {buttons [button:0x3fdcf0da1968 type: submit name:  value: ]}>,
 #<Mechanize::Form
  {name nil}
  {method "POST"}
  {action "https://pid.gree.net/"}
  {fields
   [hidden:0x3fdcf0da11e8 type: hidden name: pid_csrf[token] value: yyy]
   [hidden:0x3fdcf0da1080 type: hidden name: action value: ]
   [hidden:0x3fdcf0da0f2c type: hidden name: code value: ]
   [hidden:0x3fdcf0da0dd8 type: hidden name: app_id value: ]
   [hidden:0x3fdcf0da0c84 type: hidden name: backto value: http://gree.jp/?mode=community&act=bbs_list&community_id=xxx]}
  {radiobuttons}
  {checkboxes}
  {file_uploads}
  {buttons}>]
```