fix default prod log level

thought this was done in tobymao#10350 but apparently was missed; there are no
`LOGGER.info` calls but some `LOGGER.error` output may have ended up in the
server's stdout

lib/engine/logger.rb

@@ -11,7 +11,7 @@ def self.set_level(level, production = false)
         if level
           level.to_i
         elsif production
-          ::Logger::INFO
+          ::Logger::FATAL
         else
           ::Logger::DEBUG
         end