Use syslog instead of airbrake as syslog example

The purpose is to reduce package dependencies, fixes #809.

example_hook_test.go

@@ -2,7 +2,8 @@ package logrus_test
 
 import (
 	"github.com/sirupsen/logrus"
-	"gopkg.in/gemnasium/logrus-airbrake-hook.v2"
+	slhooks "github.com/sirupsen/logrus/hooks/syslog"
+	"log/syslog"
 	"os"
 )
 
@@ -10,7 +11,9 @@ func Example_hook() {
 	var log = logrus.New()
 	log.Formatter = new(logrus.TextFormatter)                     // default
 	log.Formatter.(*logrus.TextFormatter).DisableTimestamp = true // remove timestamp from test output
-	log.Hooks.Add(airbrake.NewHook(123, "xyz", "development"))
+	if sl, err := slhooks.NewSyslogHook("udp", "localhost:514", syslog.LOG_INFO, ""); err != nil {
+		log.Hooks.Add(sl)
+	}
 	log.Out = os.Stdout
 
 	log.WithFields(logrus.Fields{