Add eu-central-1 to list of S3 buckets.

This PR does two things.

* Adds eu-central-1 (Frankfurt) to list of S3 buckets.
* Upgrades `fog` dependency version.

I had to upgrade fog because newer S3 bucket regions (Frankfurt in particular) use a different signing algorithm that wasn’t working with previous fog, but is working with this version.

After I upgraded the fog version in `Gemfile` and ran `bundle install`, a bunch of stuff (looks like fog components/dependencies) got added to `Gemfile.lock`. I’m not really a Ruby developer so I don’t know what to make of this.