title: CarrierWave and RMagick: Using process :resize_and_pad throwing up nil is not a symbol error
body: <p>I've created an image uploader and have defined the following version within it</p>
  
  <pre><code>version :profile do
    process :resize_and_pad [100, 100]
  end
  </code></pre>
  
  <p>When I upload a file, I get the following error message:</p>
  
  <pre><code>nil is not a symbol
  </code></pre>
  
  <p>Looking at the full stacktrace, it's coming from processing.rb</p>
  
  <pre><code>carrierwave (0.6.1) lib/carrierwave/uploader/processing.rb:85:in `block in process!'
  carrierwave (0.6.1) lib/carrierwave/uploader/processing.rb:81:in `each'
  carrierwave (0.6.1) lib/carrierwave/uploader/processing.rb:81:in `process!'
  carrierwave (0.6.1) lib/carrierwave/uploader/callbacks.rb:18:in `block in with_callbacks'
  carrierwave (0.6.1) lib/carrierwave/uploader/callbacks.rb:18:in `each'
  carrierwave (0.6.1) lib/carrierwave/uploader/callbacks.rb:18:in `with_callbacks'
  carrierwave (0.6.1) lib/carrierwave/uploader/cache.rb:112:in `cache!'
  carrierwave (0.6.1) lib/carrierwave/uploader/versions.rb:230:in `block in cache_versions!'
  carrierwave (0.6.1) lib/carrierwave/uploader/versions.rb:216:in `each'
  carrierwave (0.6.1) lib/carrierwave/uploader/versions.rb:216:in `cache_versions!'
  carrierwave (0.6.1) lib/carrierwave/uploader/callbacks.rb:18:in `block in with_callbacks'
  carrierwave (0.6.1) lib/carrierwave/uploader/callbacks.rb:18:in `each'
  carrierwave (0.6.1) lib/carrierwave/uploader/callbacks.rb:18:in `with_callbacks'
  carrierwave (0.6.1) lib/carrierwave/uploader/cache.rb:112:in `cache!'
  carrierwave (0.6.1) lib/carrierwave/uploader/versions.rb:230:in `block in cache_versions!'
  carrierwave (0.6.1) lib/carrierwave/uploader/versions.rb:216:in `each'
  carrierwave (0.6.1) lib/carrierwave/uploader/versions.rb:216:in `cache_versions!'
  carrierwave (0.6.1) lib/carrierwave/uploader/callbacks.rb:18:in `block in with_callbacks'
  carrierwave (0.6.1) lib/carrierwave/uploader/callbacks.rb:18:in `each'
  carrierwave (0.6.1) lib/carrierwave/uploader/callbacks.rb:18:in `with_callbacks'
  carrierwave (0.6.1) lib/carrierwave/uploader/cache.rb:112:in `cache!'
  carrierwave (0.6.1) lib/carrierwave/mount.rb:315:in `cache'
  carrierwave (0.6.1) lib/carrierwave/mount.rb:173:in `profile_image='
  carrierwave (0.6.1) lib/carrierwave/orm/activerecord.rb:37:in `profile_image='
  activerecord (3.2.2) lib/active_record/attribute_assignment.rb:85:in `block in assign_attributes'
  activerecord (3.2.2) lib/active_record/attribute_assignment.rb:78:in `each'
  activerecord (3.2.2) lib/active_record/attribute_assignment.rb:78:in `assign_attributes'
  activerecord (3.2.2) lib/active_record/persistence.rb:212:in `block in update_attributes'
  activerecord (3.2.2) lib/active_record/transactions.rb:295:in `block in with_transaction_returning_status'
  activerecord (3.2.2) lib/active_record/connection_adapters/abstract/database_statements.rb:192:in `transaction'
  activerecord (3.2.2) lib/active_record/transactions.rb:208:in `transaction'
  activerecord (3.2.2) lib/active_record/transactions.rb:293:in `with_transaction_returning_status'
  activerecord (3.2.2) lib/active_record/persistence.rb:211:in `update_attributes'
  app/controllers/users_controller.rb:24:in `update_profile'
  </code></pre>