No need to specify aws-sdk version

Paperclip works well with aws-sdk 1 and 2, and we default to 2.

[ci skip]

README.md

@@ -577,7 +577,7 @@ You may also choose to store your files using Amazon's S3 service. To do so, inc
 the `aws-sdk` gem in your Gemfile:
 the `aws-sdk` gem in your Gemfile:
 
 
 ```ruby
 ```ruby
-gem 'aws-sdk', '~> 1.6'
+gem 'aws-sdk'
 ```
 ```
 
 
 And then you can specify using S3 from `has_attached_file`.
 And then you can specify using S3 from `has_attached_file`.