Disable OpenMP

## WHY
performance in multi core machine.

![_ _issue__34_ _wantedly_infrastructure](https://cloud.githubusercontent.com/assets/261700/5469986/dcc26fdc-8625-11e4-92bf-82128bd304b7.png)

Disable OpenMP

![i-e4e9dcfd___datadog](https://cloud.githubusercontent.com/assets/261700/5469961/80cb8632-8625-11e4-935b-f7a3a29ecd6e.png)


## TODO

* [ ] add graph for CPU usage to compare

## REF

* [Parallel Execution With OpenMP](http://www.imagemagick.org/script/openmp.php)