Fix unrecognized image format error when rendering invoice logo

solidusio/solidus#2995 replaced Solidus' .png logo with a .svg version,
a format that Prawn does not support. This patch re-introduces the former
logo and uses it if no custom configuration is provided.

app/models/spree/print_invoice_configuration.rb

@@ -2,7 +2,7 @@ module Spree
   class PrintInvoiceConfiguration < Preferences::Configuration
 
     preference :print_invoice_next_number, :integer, :default => nil
-    preference :print_invoice_logo_path, :string, :default => Spree::Config[:admin_interface_logo]
+    preference :print_invoice_logo_path, :string, :default => Spree::Config[:admin_interface_logo].end_with?('.svg') ? 'logo/solidus_logo.png' : Spree::Config[:admin_interface_logo]
     preference :print_invoice_logo_scale, :integer, :default => 50
     preference :print_invoice_font_face, :string, :default => 'Helvetica'
     preference :print_buttons, :string, :default => 'invoice'
@@ -17,6 +17,5 @@ def increase_invoice_number
       set_preference(:print_invoice_next_number, current_invoice_number + 1)
       current_invoice_number
     end
-
   end
 end