Fix requirement of image_tag in zoomable_image_tag

On a vanilla machine installing the blog via the fork/pull request method, `rake generate` produces the following error out of the box: 
`zoomable_image_tag.rb:20:in <module:Jekyll>': uninitialized constant Jekyll::ImageTag (NameError)`

This is a fix for the missing `ImageTag` plugin that `zoomable_image_tag` plugin requires