Lock `aws-sdk` version in the readme

README.md

@@ -176,7 +176,7 @@ safer choice for the default file store._
 You may also choose to store your files using Amazon's S3 service. To do so, include
 the `aws-sdk` gem in your Gemfile:
 
-    gem 'aws-sdk'
+    gem 'aws-sdk', '~> 1.3.4'
 
 And then you can specify using S3 from `has_attached_file`.
 You can find more information about configuring and using S3 storage in