Ast override makes is possible to bypass

`Parser` should be in the forbidden list, because it is still possible to bypass the protection by monkey-patching it:
```
class ::Parser::AST::Processor
	def on_const(node)
		node = [:String]
	end
end
conn = PG::Connection.open(:dbname => 'rails_edge_development')
conn.exec "delete from console1984_sessions"
```