Fix wrong Downtime Calculation

The downtime was calculated wrong because the time when the latest
failure oocured was subtraced of the time when the latest failure
occured. And this will be zero.

Now it subtractes from the Date of the latest failure the Date of the
latest successfull hit.

Signed-off-by: Emanuel Bennici <benniciemanuel78@gmail.com>

core/services.go

@@ -260,7 +260,7 @@ func (s *Service) Downtime() time.Duration {
 	if len(hits) == 0 {
 		return time.Now().UTC().Sub(fail.CreatedAt.UTC())
 	}
-	since := fail.CreatedAt.UTC().Sub(fail.CreatedAt.UTC())
+	since := fail.CreatedAt.UTC().Sub(hits[0].CreatedAt.UTC())
 	return since
 }