Remove pathname require from ruby_project.rb

Achieved by implement ascend simply ourselves.
These specs all follow the same form.  I think it's worth extracting a common helper method:

```ruby
def expect_ascend(source_path, *yielded_paths)
  allow(File).to receive(:expand_path).with('.') { source_path }
  expect { |probe|
    subject.ascend_until(&probe)
  }.to yield_successive_args(*yielded_paths)
end

# call like:
expect_ascend("/var//ponies/", "/var/ponies", "/var", "/")
```

Thoughts?