Bug 27485: Rename system preference gist to TaxRates

Bug 27485: Rename system preference gist to TaxRates

Test Plan:

1. Create a vendor
2. Create a basket
3. Add an order to the basket (note the existence of the option 'gist'
4  Apply patch
5. Create another order (note the existence of the option 'TaxRates'
    instead)
6. Run 'prove t/Prices.t' to confirm the tests were successful
7. If everything above is correct then patch was successful

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>