Copy .rubocop.yml into sandbox

Similar to the copying of `.cane` file, this allows customizing the rubocop ruby style checking rules.

At present, the default rubocop rules don't exclude `vendor/bundle` which means when running strainer on my cookbook repo on travis-ci (and presumably other CI's), rubocop is checking all gems installed from the Gemfile. I'll see about proposing changes to rubocops defaults, but aside from that it would be preferable to also copy across any other customizations