only require fog providers needed for tests

paperclip.gemspec

@@ -43,7 +43,8 @@ Gem::Specification.new do |s|
   # Ruby version < 1.9.3 can't install capybara > 2.0.3.
   s.add_development_dependency('capybara')
   s.add_development_dependency('bundler')
-  s.add_development_dependency('fog', '~> 1.0')
+  s.add_development_dependency('fog-aws')
+  s.add_development_dependency('fog-local')
   s.add_development_dependency('launchy')
   s.add_development_dependency('rake')
   s.add_development_dependency('fakeweb')

spec/paperclip/storage/fog_spec.rb

 require 'spec_helper'
-require 'fog'
+require 'fog/aws'
+require 'fog/local'
 require 'timecop'
 
 describe Paperclip::Storage::Fog do