Fix bug with MFA codes that look like invalid octal numbers

Always treat MFA code as a base 10 integer. Otherwise if you enter codes like 099999, it will try to parse it as an
octal number and raise an ArgumentError in sprintf.

```
>> sprintf('%06d', '099999')
ArgumentError: invalid value for Integer(): "099999"
	from (irb):12:in `sprintf'
	from (irb):12
	from /home/andy/.rbenv/versions/1.9/bin/irb:12:in `<main>'
```