Modified log4go import path

Google code stopped service, so `make release-server` failed:
```
package code.google.com/p/log4go: unable to detect version control system
```

src/ngrok/log/logger.go

@@ -1,7 +1,7 @@
 package log
 
 import (
-	log "code.google.com/p/log4go"
+	log "github.com/alecthomas/log4go"
 	"fmt"
 )