sources: remove image_url method from base strategy.

Remove the `image_url` method from source strategies. This method would
return only the first image if a source had multiple images. The
`image_urls` method should be used instead. Tests were the main place
that still used `image_url` instead of `image_urls`.

Also make post replacements return an error if replacing with a source
that contains multiple images, instead of just blindly replacing the
post with the first image in the source.

test/unit/downloads/pixiv_test.rb

@@ -131,7 +131,7 @@ module Downloads
     context "An ugoira site for pixiv" do
       should "capture the data" do
         @strategy = Sources::Strategies.find("http://www.pixiv.net/member_illust.php?mode=medium&illust_id=62247364")
-        media_file = @strategy.download_file!
+        media_file = @strategy.download_file!(@strategy.image_urls.sole)
 
         assert_equal(2, media_file.frame_data.size)
         assert_equal([{"file" => "000000.jpg", "delay" => 125}, {"file" => "000001.jpg", "delay" => 125}], media_file.frame_data)