Strip non digit from zip for elavon

#### Problem
https://github.com/Shopify/active_merchant/issues/1445

#### Changes
Remove any non digit characters from ```avs_zip``` and ```slice``` to 9 instead of 10. Intuitively, you'd expect ```ship_to_zip``` to also have the same requirements, but nope, according to [docs](https://www.myvirtualmerchant.com/VirtualMerchant/download/developerGuide.pdf) p.235-245 only ```avs_zip``` is limited to 9 digits, ```ship_to_zip``` can be 10.

#### Review
@bizla @ivanfer
Uhhhh, this gateway supports [other countries, including Canada](https://github.com/Shopify/active_merchant/blob/master/lib/active_merchant/billing/gateways/elavon.rb#L39) so this will clobber postal codes. Is `:avs_zip` a US-only thing?