Use a Hash instead of a String for image.thumbnail

This is necessary or you'll encounter this error:
can't convert String into Hash

The possible options are:
options = { :geometry, :strip }

Source: https://github.com/refinery/refinerycms/blob/master/images/app/models/refinery/image.rb#L40
Merge branch 'master' into patch-1
Merge pull request #96 from givanse/patch-1

Use a Hash instead of a String for image.thumbnail()