Ensure timeout is a number

lib/github/markup/command_implementation.rb

@@ -42,7 +42,7 @@ module GitHub
       end
 
       def timeout_in_seconds
-        ENV.fetch('GITLAB_MARKUP_TIMEOUT', DEFAULT_GITLAB_MARKUP_TIMEOUT)
+        ENV.fetch('GITLAB_MARKUP_TIMEOUT', DEFAULT_GITLAB_MARKUP_TIMEOUT).to_i
       end
 
       def prepend_command_timeout_prefix(command)