Fix docs deployment
Fix docs deployment

This fixes issue with `s3_website` while pushing Jekyll site to
staging/production:
```
[fail] Could not load the site: Failed to parse ERB in staging/s3_website.yml:
(SyntaxError) /usr/local/lib/ruby/site_ruby/2.5.0/rubygems/core_ext/kernel_warn.rb:15: syntax error, unexpected tLABEL
module_function define_method(:warn) {|*messages, uplevel: nil|
```

Examples of failed builds:
- https://circleci.com/gh/airbrake/airbrake-docs/1076
- https://circleci.com/gh/airbrake/airbrake-docs/1070

The fix is to tell the `s3_website` gem to only download the `.jar` file,
then manually invoke it.

There is an opened related issue in https://github.com/laurilehmijoki/s3_website/issues/323.

airbrake/airbrake-docs,airbrake/airbrake-docs,airbrake/airbrake-docs,airbrake/airbrake-docs