Simplify the expected return on egithub_webhook

[elbrujohalcon]

Instead of requesting the callback module to find out the commit_id and the position let the module just return filename, line_number and text and do the rest of the processing inside egithub_webhook (use the current implementation in elvis_webhook as a reference).

[elbrujohalcon]

Also, make sure CommitId is always a binary and not a string.