Dereference pointer in `Requried` Validator

in [validator.go][https://github.com/jamieomatthews/validation/blob/master/validator.go#21]
```golang
//obj is pointer
func (r Required) IsValid(obj interface{}) bool {
if obj == nil {
        return false
}

//type assert always fail
if str, ok := obj.(string); ok {
        return len(str) > 0
}
// ...

//and Required always return true
return true
}
```