Update uploaders to use 'aws' rather than 's3'

This change should fix the application crashing in staging. Carrierwave didn't seem to like :s3 as a storage engine, and having reviewed the docs I think this is the right change.

Author: jonodrew

app/uploaders/avatar_uploader.rb

@@ -1,7 +1,7 @@
 # encoding: utf-8
 
 class AvatarUploader < CarrierWave::Uploader::Base
-  storage :s3 if Rails.env.production?
+  storage :aws if Rails.env.production?
 
   include CarrierWave::MiniMagick
 

app/uploaders/image_uploader.rb

@@ -1,5 +1,5 @@
 class ImageUploader < CarrierWave::Uploader::Base
-  storage :s3 if Rails.env.production?
+  storage :aws if Rails.env.production?
 
   # Include RMagick or MiniMagick support:
   include CarrierWave::MiniMagick