parse the correct account number from deutsche bank files

Hi, I've now used my extension for parsing files from deutsche bank the first time with real data and noticed that the account_no and bank_code in the bank-statement are swapped.

That's because the deutsche bank uses the following notation in `:25:` tag:

```
:25:(bank_code)/(account_no)
```

I updated my spec to look for the correct number and changed the implementation a bit. All other implementations aren't affected because this notation seems either to be a special case in germany or for the deutsche bank :grinning: