Travis: activate CHECK_LEAKS for 2.4.0
Travis: activate CHECK_LEAKS for 2.4.0

nobu/rubyspec,ruby/spec,eregon/rubyspec,nobu/rubyspec,ruby/rubyspec,eregon/rubyspec,ruby/rubyspec,sgarciac/spec,nobu/rubyspec,ruby/spec,sgarciac/spec,sgarciac/spec,eregon/rubyspec,ruby/spec