update references to old certificates on smtp & fast server

lib/postal/fast_server/client.rb

@@ -138,10 +138,10 @@ module Postal
           end
 
           if ssl_context.cert.nil?
-            certs = Postal.ssl_certificates
+            certs = Postal.smtp_certificates
             ssl_context.cert = certs.shift
             ssl_context.extra_chain_cert = certs
-            ssl_context.key  = Postal.signing_key
+            ssl_context.key  = Postal.smtp_private_key
           end
 
           ssl_context.ssl_version = "SSLv23"

lib/postal/smtp_server/server.rb

@@ -37,10 +37,10 @@ module Postal
       def ssl_context
         @ssl_context ||= begin
           ssl_context      = OpenSSL::SSL::SSLContext.new
-          certs            =  Postal.ssl_certificates
+          certs            =  Postal.smtp_certificates
           ssl_context.cert = certs.shift
           ssl_context.extra_chain_cert = certs
-          ssl_context.key  = Postal.signing_key
+          ssl_context.key  = Postal.smtp_private_key
           ssl_context.ssl_version = "SSLv23"
           ssl_context
         end