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
 You may also choose to store your files using Amazon's S3 service. To do so, include
 the `aws-sdk` gem in your Gemfile:
 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`.
 And then you can specify using S3 from `has_attached_file`.
 You can find more information about configuring and using S3 storage in
 You can find more information about configuring and using S3 storage in