fix ean check digit

This is the problem behind #219. #219 is ok, but ean check digit algorithm was wrong.

Before:
```
>> 5000.times.map { Faker::Code.ean(13).length == 13 }.uniq
=> [true, false]
>> 5000.times.map { Faker::Code.ean(8).length == 8 }.uniq
=> [true, false]
```

After:
```
>> 5000.times.map { Faker::Code.ean(13).length == 13 }.uniq
=> [true]
>> 5000.times.map { Faker::Code.ean(8).length == 8 }.uniq
=> [true]
```