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.go

@@ -1,12 +1,13 @@
 // +build !windows,!nacl,!plan9
 
-package logrus_syslog
+package syslog
 
 import (
 	"fmt"
-	"github.com/sirupsen/logrus"
 	"log/syslog"
 	"os"
+
+	"github.com/sirupsen/logrus"
 )
 
 // SyslogHook to send logs via syslog.