Rename credit card icon

We ran into an issue with a naming problem for the dinersclub.png cc icon. We got the following error:
```
Completed 500 Internal Server Error in 311ms

ActionView::Template::Error (credit_cards/icons/diners_club.png isn't precompiled):
    39:       <% unless order.credit_cards.empty? %>
    40:         <span class="cc-type">
    41:           <% unless (cc_type = order.credit_cards.first.cc_type).blank? %>
    42:             <%= image_tag "credit_cards/icons/#{order.credit_cards.first.cc_type}.png" %>
    43:           <% end %>
    44:           <%= t(:ending_in)%> <%= order.credit_cards.first.last_digits %>
    45:         </span>
```

The error stems from the fact that diners club png is actually named `dinersclub.png` while the naming of those icons is [generated here](https://github.com/Shopify/active_merchant/blob/master/lib/active_merchant/billing/credit_card_methods.rb#L75) from the Active Merchant gem. As you can see, the list it pulls names from uses an underscore for diners_club.

I haven't provided any test coverage, but I figured an image re-name wouldn't require any. Let me know if that's incorrect. 

Finally, this should probably be changed on 1-2-stable, 1-3-stable, etc.