Fix method redefined warning for support_email (#1214)

`mattr_accessor :support_email` creates a `support_email=` writer, but
we immediately redefine it to add Mail::Address casting. This triggers
Ruby's "method redefined" warning when running with warnings enabled.

Use `mattr_reader` instead since we define the writer manually.

Author: joemasilotti

lib/pay.rb

@@ -38,7 +38,7 @@ module Billable
   mattr_accessor :business_address
   mattr_accessor :business_name
   mattr_accessor :business_logo
-  mattr_accessor :support_email
+  mattr_reader :support_email
 
   def self.support_email=(value)
     @@support_email = value.is_a?(::Mail::Address) ? value : ::Mail::Address.new(value)