Updated gemspec to fix issue #1729

Updated gemspec to require aws-sdk gem ~> 1.6

paperclip.gemspec

@@ -32,7 +32,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency('rspec')
   s.add_development_dependency('appraisal')
   s.add_development_dependency('mocha')
-  s.add_development_dependency('aws-sdk', '~> 1.5')
+  s.add_development_dependency('aws-sdk', '~> 1.6')
   s.add_development_dependency('bourne')
   s.add_development_dependency('cucumber', '~> 1.3.18')
   s.add_development_dependency('aruba')