Fix _parse_jenkins_json to handle application/json content-type.

With Jenkins Notification plugin 1.5, the content-type appears to be set
correctly to application/json. This change should work with previous versions
that set it to application/x-www-form-urlencoded.
Merge pull request #17 from steveprentice/jenkins_notification_fix

Fix _parse_jenkins_json to handle application/json content-type.