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.
Refer to Fog in the global namespace
Only require generator_spec when needed

generator_spec has a dependency on Railties, which defines the Rails
module, apparently without defining the .env method that is needed
when testing on Rails 4.1.

See https://github.com/rails/rails/blob/v4.1.0/activerecord/lib/active_record/connection_handling.rb#L3
Fixes deprecated File.exists?

http://www.ruby-doc.org/core-2.1.1/File.html#method-c-exist-3F
Merge pull request #1374 from futhr/feature/deprecation-fixup

Fixes deprecated File.exists?
Merge pull request #1369 from bquorning/bquorning/fix-4.1-specs

Only require generator_spec when needed
Merge pull request #1364 from bquorning/less-fog

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