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

The improvement trigger is only triggered when there are LESS failing tests. It implies being UNSTABLE or STILL UNSTABLE. Otherwise it would be SUCCESS or FIXED.
This modification makes the IMPROVEMENT trigger overriding the UNSTABLE or STILL UNSTABLE triggers.
Merge pull request #72 from stephanebruckert/patch-3

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