Fix: authentication failed on www.yammer.com

Created by: tach

I cannot use omniauth-yammer for www.yammer.com OAuth2 autentication but it fails. Because yammer's access_token value is like hash not a string as follows

https://developer.yammer.com/api/oauth2.html "access_token": { "view_subscriptions": true, "expires_at": null, "authorized_at": "2011/04/06 16:25:46 +0000", "modify_subscriptions": true, "modify_messages": true, "network_permalink": "yammer-inc.com", "view_members": true, "view_tags": true, "network_id": 155465488, "user_id": 1014216, "view_groups": true, "token": "ajsdfiasd7f6asdf8o", "network_name": "Yammer", "view_messages": true, "created_at": "2011/04/06 16:25:46 +0000" },

I'm not familiar with OAuth2 specification but this patch fix this issue.

And also I create pull request to omniauth-yammer for removing overrode build_access_token.