change default v7 to v5

[reckless-huang]

i want change default v7 struct to v5 ,use NewAdapterByDBWithCustomTable is not work

type CasbinRule struct {
	ID    uint   `gorm:"primaryKey;autoIncrement"`
	Ptype string `gorm:"size:100;uniqueIndex:unique_index"`
	V0    string `gorm:"size:100;uniqueIndex:unique_index"`
	V1    string `gorm:"size:100;uniqueIndex:unique_index"`
	V2    string `gorm:"size:100;uniqueIndex:unique_index"`
	V3    string `gorm:"size:100;uniqueIndex:unique_index"`
	V4    string `gorm:"size:100;uniqueIndex:unique_index"`
	V5    string `gorm:"size:100;uniqueIndex:unique_index"`
}

func InitCasbin() {
	mysqlc := common.CONFIG.Mysqls[0]
	host, port, user, password, database := mysqlc.Host, mysqlc.Port, mysqlc.User, mysqlc.Password, mysqlc.Database
	dsn := fmt.Sprintf("%s:%s@tcp(%s:%d)/%s?charset=utf8mb4&parseTime=True&loc=Local", user, password, host, port, database)
	db, err := gorm.Open(mysql.Open(dsn), &gorm.Config{})
	if err != nil {
		common.Log.Error(err.Error())
		return
	}
	gormAdapter.TurnOffAutoMigrate(db)
	a, err := gormAdapter.NewAdapterByDBWithCustomTable(db, &CasbinRule{}, "casbin_rule")
	if err != nil {
		common.Log.Error(err.Error())
		return
	}
	//a, _ := gormAdapter.NewAdapter("mysql", dsn, true)
	e, err := casbin.NewEnforcer(constants.CasbinModel, a)
	if err != nil {
		common.Log.Error(err.Error())
		return
	}
	e.AddFunction("ParamsMatch", ParamsMatchFunc)
```
can you help me , is err here 
Error 1054: Unknown column 'v6' in 'field list'
[73.947ms] [rows:0] INSERT INTO `casbin_rule` (`ptype`,`v0`,`v1`,`v2`,`v3`,`v4`,`v5`,`v6`,`v7`) VALUES ('g','develop','base','','','','','','')

[casbin-bot]

@tangyang9464 @JalinWang @imp2002

[hsluoyz]

We added v6 and v7 because of this issue: #151

@zz541843 can we revert the PR?

[hsluoyz]

@tangyang9464 what's your opinion?

@JalinWang

[JalinWang]

Maybe we need to extend #168 to a more flexible schema?

[hsluoyz]

@JalinWang plz revert PR: #157

[tangyang9464]

Maybe we need to extend #168 to a more flexible schema?

@JalinWang I think so.

[tangyang9464]

@JalinWang plz revert PR: #157

@hsluoyz I think we should extend #168 to support v7 instead of revert

[reckless-huang]

@JalinWang plz revert PR: #157

@hsluoyz I think we should extend #168 to support v7 instead of revert

every thanks , what time for merge #168

[hsluoyz]

@tangyang9464 @JalinWang I still think we should make it simple and just use up to v5 element. The user can fork the library and extend elements as they want. Supporting customizing it will take a lot of efforts for our maintainers team. We may need two weeks to just finish: #180 and this is only one adapter. Remember we have 20+ adapters only for Golang and it will be a disaster to support them all. Not to mention we have 10 more other languages, Java, Python, Node.js, PHP, .NET and each language will have a bunch of adapters. This is something we cannot afford to do with the current development team. Moreover, I don't think it's very meaningful. Just fork it and extend to v7 or v17. It's already so easy. Don't waste tons of our development team's efforts to just fix this small issue.