Sends notifications for completed events to slack

[kenXengineering]

The slack notifier is currently sending notifications for completed events. Regex is wrong.

ec2EventNotifier/notifiers/slack.go

Line 58 in 074733a

var completedRegexp = regexp.MustCompile("^Completed")

Example description:
[Completed] scheduled reboot

[kenXengineering]

Should be fixed in b2d6557.