[ENSCORESW-2612]. Send notification only when build status changes, on_success...

[ENSCORESW-2612]. Send notification only when build status changes, on_success defaults to 'change' accoring to documentation.

.travis.yml

@@ -56,7 +56,7 @@ matrix:
 
 notifications:
   email:
-    on_success: always
-    on_failure: always
+    on_failure: change
   slack:
     secure: gTcWzQsVHKPxRJ1u0U2l2+QVujmG1uFW44kAO3l46V5Im5uLTOLeM20ykk6ox4kaPHLs5ky7NGzcJmTDLaHBKjI4Sa34l4tJd0lhkv+hKj9TSv5pDDCdUX8v869Nm8LqJKh26HqvVcN7eHFRatLD/tfYvvkaP1rLLhwLLdr7czQ=
+    on_failure: change