fix(notifications): Need to maintain enum order (#589)

* fix(notifications):  Need to maintain enum order

HIPCHAT was removed from the Enum here, but this enum is duplicated in
other microservices (for example, in Orca,
orca-echo/src/main/groovy/com/netflix/spinnaker/orca/echo/EchoService.groovy
has a copy of the list)

This is a quick fix to repair the numbering scheme.  This should
probably be fully refactored so the enum is either a) specifying actual
values for each type so they maintain consistency going forward, b)
unify in kork somehow, or c) both.

* Keep old enum name, update comment, per request.

echo-notifications/src/main/groovy/com/netflix/spinnaker/echo/api/Notification.groovy

@@ -38,6 +38,7 @@ class Notification {
     EMAIL,
     GITHUB_STATUS,
     GOOGLECHAT,
+    HIPCHAT, // DEPRECATED/UNUSED; support was removed in commit d175913ab
     JIRA,
     PAGER_DUTY,
     PUBSUB,