This is a requirement to run the tests.

````Ruby
thin_http_server.rb:20:in `require': cannot load such file -- rack (LoadError) 
````

If I added the `rack` gem then:

````Ruby
thin_http_server.rb:21:in `require': cannot load such file -- thin (LoadError)
````

`rack` is already a dependency of `thin`, so we just need to add the `thin` gem to fix it.

`````Ruby
# Running tests:

.....................................................................................................................................

Finished tests in 10.066845s, 13.2117 tests/s, 25.0327 assertions/s.

133 tests, 252 assertions, 0 failures, 0 errors, 0 skips
`````