Ruby: fixed crash on thread start.

Ruby threads need to be created with GVL; otherwise, an attempt to access
locked resources may occur, causing a crash.

The issue was occasionally reproduced on Ubuntu 18.04 with Ruby 2.5.1
while running test_ruby_application_threads.

docs/changes.xml

@@ -22,6 +22,12 @@ in PHP module.
 </para>
 </change>
 
+<change type="bugfix">
+<para>
+Ruby apps with configured mutlithreading could crash on start under load.
+</para>
+</change>
+
 </changes>