Simple class definition does not work

A very basic definition of a service class that inherits from Servitium::Service does not work.

```
require "servitium"

class TestService < Servitium::Service
  attribute :a
end

puts TestService
```

expected result:

```
TestService
```

actual result:

```
test.rb:4:in '<class:TestService>': undefined method 'attribute' for class TestService (NoMethodError)

  attribute :a
  ^^^^^^^^^
```

