ensure cef_version is an integer as described in standard

otherwise throw exception/tag as parse failure

see more context in https://github.com/logstash-plugins/logstash-codec-cef/pull/26#pullrequestreview-3551244

cef specification https://www.protect724.hpe.com/servlet/JiveServlet/downloadBody/1072-102-9-20354/CommonEventFormatv23.pdf
