Update linux upstart detection to support RHEL6 (#92)

* Update linux upstart detection to support RHEL6

I've added additional detection logic to the isUpstart function, that should validate the existence of upstart on a RHEL 6 system.

* Remove debug message

service_upstart_linux.go

@@ -17,6 +17,13 @@ func isUpstart() bool {
 	if _, err := os.Stat("/sbin/upstart-udev-bridge"); err == nil {
 		return true
 	}
+	if _, err := os.Stat("/sbin/init"); err == nil {
+		if out, err := exec.Command("/sbin/init", "--version").Output(); err == nil {
+			if strings.Contains(string(out),"init (upstart") {
+				return true
+			}
+		}
+	}
 	return false
 }