rename logrus_syslog package to syslog

Package names shouldn't be using underscores. There is also nothing
wrong with naming this package `syslog`. People can include it with any
name they wish.

hooks/syslog/syslog_test.go

@@ -1,9 +1,10 @@
-package logrus_syslog
+package syslog
 
 import (
-	"github.com/sirupsen/logrus"
 	"log/syslog"
 	"testing"
+
+	"github.com/sirupsen/logrus"
 )
 
 func TestLocalhostAddAndPrint(t *testing.T) {