Don't send X-Rollbar-Access-Token header.

It seems that the payload already have the token of the project that generated the exception. We can see that if we are using eventmachine this header is no sent, so I supose it's not really needed.

This fix issues when the project sending the payload is not the project that generated the exception. This is the case when we enqueue a job to send the report but the job is processed by a resque process in other project.