Add `ar_internal_metadata` table to default value of `skip_tables`

[yujideveloper]

ar_internal_metadata table was added at Rails 5.0 (ref. rails/rails#22967) for prevent destructive action on production database. It is also created on other rails environment other than production(e.g. development/test). So, I added it to default value of skip_tables.

[yujideveloper]

@thewoolleyman What do you think this?

[thewoolleyman]

Looks straightforward, I'll merge it.

If anyone notices this breaks something, give me a heads up.