ensure file_name is set if flatten_zip is false when zipping additional ...

With the zip task, if you set flatten_zip to false and you include additional files, it errors with:


    add failed. Entry  already exists

This is because the file_name is only set to a value if @flatten_zip:

```ruby
file_name = file_path.split('/').last if @flatten_zip
```

I have fixed this with a simple one line change. My usage is:

```ruby
  zip :zip do |zip|
    # Currently broken in albacore need to submit fix.
    zip.flatten_zip = false
    zip.directories_to_zip ".bundle", "parameters", "build-scripts"
    zip.additional_files = ["configatron.env.yml", "Gemfile", "Gemfile.lock", "rakefile.rb"]
    zip.output_file = 'rake-package.zip'
    zip.output_path = File.dirname(__FILE__)
  end
```