Add reverse proxy caching

There is a `<% end -%>` missing to closes the `if`.

Also what do you think about only testing for presence of the `node['nginx']['reverse_proxy_caching']` attribute, instead of equality testing for the value `"on"`(like done with the `<% if node['nginx']['enable_rate_limiting'] -%>` below and [corresponding attribute](https://github.com/miketheman/nginx/blob/e128a0ad7322b0d406857aac5efc7a1c1cb824cd/attributes/rate_limiting.rb#L20)).