Fix Darwin specific issue with `--parents`

Fix for a Darwin/OSX specific issue for the build rake tasks.

```bash
bundle exec rake build:tar
Compiling assets and templates into ./app
cp -r --parents layouts/govuk_template.html.erb /Users/petems/Projects/govuk_template/app/views
Error copying views:
cp: illegal option -- -
usage: cp [-R [-H | -L | -P]] [-fi | -n] [-apvX] source_file target_file
       cp [-R [-H | -L | -P]] [-fi | -n] [-apvX] source_file ... target_directory
```

`--parents` isn't a part of the OSX cp library, so the rake task will fail on OSX. Rsync provides the same functionality for Darwin, so changed (this could be changed into `ditto` if needed) :+1: