Fix an issue with thousand separators in IPay88

## Problem
IPay88 returns the amount with thousand comma separators. According to the [template](https://github.com/Shopify/offsite_payments/blob/5b9453e679c3b1e5e39a3e0e29fc7e8ae1a13fdf/generators/templates/integration.rb#L72-75) `gross` should be the money amount we received in X.2 decimal. For amounts higher than $999, IPay88 adds a comma separator which doesn't obey the `X.2` decimal format.

## Solution
Remove commas that are followed by 3 digits in the amount.

@odorcicd @bslobodin 
/cc @girasquid @andrewdunbar 
Testing for `nil` is actually done in `test_unsuccessful_acknowledge_due_to_missing_amount`. But I'll test for stripping commas correctly.