We don't need to require all of Fog using ~> 1.20

With the release of Fog v1.20.0, it is possible to only require the specific parts of Fog that you need, e.g. `require "fog/aws/storage"` which will load 36 classes, whereas `require "fog"` loads 698 classes in my tests.

See discussion in #1337.

cc @bensie, @elight