Bypass for newer OpenSSL versions (#999)

lib/postal/fast_server/client.rb

@@ -152,9 +152,12 @@ module Postal
                                 OpenSSL::SSL::OP_NO_SSLv3 |
                                 OpenSSL::SSL::OP_NO_COMPRESSION |
                                 OpenSSL::SSL::OP_CIPHER_SERVER_PREFERENCE
+
+          if ssl_context.respond_to?('tmp_ecdh_callback=')
             ssl_context.tmp_ecdh_callback = Proc.new do |*a|
               OpenSSL::PKey::EC.new("prime256v1")
             end
+          end
 
           unless domain_name
             ssl_context.servername_cb = Proc.new do |ctx, hostname|