[JENKINS-18609] Triggers two mails: UNSTABLE and REGRESSION

https://issues.jenkins-ci.org/browse/JENKINS-18609?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel
[JENKINS-18609] Triggers two mails: UNSTABLE and REGRESSION

Also needs to override the StillUnstable trigger.
Merge pull request #71 from stephanebruckert/patch-2

[JENKINS-18609] Triggers two mails: UNSTABLE and REGRESSION