gems: drop ruby-imagespec gem.

* Use libvips instead of ruby-imagespec for reading dimensions of jpeg, png, and gif files.
* Copy the code for reading the dimensions of flash files from ruby-imagespec.

Fixes an incompatibility between ruby-imagespec and the rubocop gem that
prevented us from including rubocop in the Gemfile.

app/logical/media_file/image.rb

@@ -1,6 +1,9 @@
 class MediaFile::Image < MediaFile
   def dimensions
-    image_size = ImageSpec.new(file.path)
-    [image_size.width, image_size.height]
+    image.size
+  end
+
+  def image
+    @image ||= Vips::Image.new_from_file(file.path)
   end
 end