title: ActiveStorage wont crop variants
body: <p>I'm migrating my rails app from paperclip to ActiveStorage and it just won't accept the <code>crop</code> argument in a variant</p>
  
  <p>this line:</p>
  
  <pre><code>@user.image.variant(crop: [180,135])
  </code></pre>
  
  <p>cause this error:</p>
  
  <pre><code>Errno::ENOENT (No such file or directory @ rb_sysopen - /var/folders/dd/dy3xgqrs2vv6h97ckrtmrb4m0000gn/T/mini_magick20180526-14598-njz21n.jpg):
  
  activestorage (5.2.0) app/models/active_storage/variant.rb:130:in `initialize'
  activestorage (5.2.0) app/models/active_storage/variant.rb:130:in `open'
  activestorage (5.2.0) app/models/active_storage/variant.rb:130:in `upload'
  activestorage (5.2.0) app/models/active_storage/variant.rb:88:in `block in process'
  activestorage (5.2.0) app/models/active_storage/variant.rb:110:in `open_image'
  activestorage (5.2.0) app/models/active_storage/variant.rb:85:in `process'
  activestorage (5.2.0) app/models/active_storage/variant.rb:53:in `processed'
  activestorage (5.2.0) app/controllers/active_storage/representations_controller.rb:12:in `show'
  </code></pre>
  
  <p>while eg. this works:</p>
  
  <pre><code>@user.image.variant(resize: '180x135')
  </code></pre>