Workaround for 'cannot load such file -- unicorn/worker_killer' errors.

This defines a simple index file at the gem's root directory that you can point your Gemfile by using:

```gem unicorn-worker-killer, require: 'unicorn_worker_killer```

This seems to satisfy config.ru when it can't load the file from its original location, as seen in https://github.com/kzk/unicorn-worker-killer/issues/35.

The require in config.ru needs to be updated to

```require 'unicorn_worker_killer'```