

<!-- Thank you for submitting an issue!. 
     Please fill following if applicable, delete otherwise: -->
```
* Operating system:                linux / mac / win
* Ruby implementation:             Ruby / JRuby / TruffleRuby / Rubinius
* `concurrent-ruby` version:       x.y.z
* `concurrent-ruby-ext` installed: yes / no
* `concurrent-ruby-edge` used:     yes / no
```
