[WIP] Issue #53 Fix hook.rb not to make defunct process

#53 

I'm planning to rewrite pooi itself and `hook.rb` as follows:

* [x] Make pool enable to work with remote API
* [ ] Create another Dockerfile to create pool's base like: [ainoya/apache-mod-mruby Repository | Docker Hub Registry - Repositories of Docker Images](https://registry.hub.docker.com/u/ainoya/apache-mod-mruby/)
  * To use [mattn/mruby-http](https://github.com/mattn/mruby-http) to communicate with Docker remote API, mruby needs to build with that mgem. `apache-mod-mruby` is for general purpose so that I'm going to another Dockerfile for it.
* [ ] Remove `` Kernel.` ``
* [ ] Rewrite `hook.rb` to inspect status of containers to access Docker remote API