Make it possible to run rspec with single spec file

If you try to run rspec with a single spec file you get:

```
bundle exec rspec spec/aruba/api_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
  An error occurred in an after hook
    NoMethodError: undefined method `stop_processes!' for nil:NilClass
    occurred at /home/xgvndeg/data/clones/aruba/spec/aruba/api_spec.rb:339:in `block (4 levels) in <top (required)>'

F
  An
[...]
```
This PR fixes this by including the `Aruby::Api`. After that change you can run `bundle exec rspec spec/aruba/api_spec.rb` without any error.