fix: upgrade middleman-s3_sync to fix deploys

[ovasdi]

This PR fixes (currently broken) deploy job that is throwing the following error:

bundler: failed to load command: middleman (/root/project/vendor/bundle/ruby/2.6.0/bin/middleman)
LoadError: cannot load such file -- xmlrpc/client

In ruby v2.4.0 the xmlrpc lib was removed from stdlib and extracted into a gem. This adds that gem as a dependency and also updates the .ruby-version to match the version used in CI config.

[ovasdi]

@joeyAghion Thanks to your comment I took a closer look and noticed that:

• The fog gem v1.27.0 is not required anywhere in the project.
• I then noticed that the currently installed version of middleman-s3_sync v3.0.40 gem depends on fog, which depends on xmlrpc, but a more recent version of middleman-s3_sync has been updated to use fog-aws, likely following this recommendation.

I decided to:

• Remove the fog gem since its not required.
• Upgrade the _middleman-s3sync instead of including xmlrpc gem to fix deploys. Running bundle exec middleman build locally still succeeds.

[joeyAghion]

Nice--that's even better.