Add rack-health.rb to help bundler load rack/health.rb

### before
```ruby
gem "rack-health", require: "rack/health"
```

### after
```ruby
gem "rack-health"
```

### testing
Just confirmed by the following command:

```sh
# shell-command
$ cd /path/to/rack-health
$ ruby -Ilib -rrack-health -e "p defined?(Rack::Health)"
"constant"
```