Update to fog 1.25 gem to support AWS4.

Firstly, this is an awesome gem.
For fog, see https://github.com/fog/fog/blob/master/CHANGELOG.md : version 1.25 is needed if you want to use the new locations like Frankfurt, as they ONLY support AWS v4 and up.
Locally I changed my gemfile.lock to test with the backup gem and a Frankfurt Amazon bucket. Everything works as expected.