Some imgur URLS have the extension ".gifv" and show up as a gif,
even though they're actually supposed to be mp4 videos. Imgur
serves all videos/gifs as both .gif and .mp4. The image dict has
a key "prefer_video" to distinguish the two. This commit
overrides the .gif extension if "prefer_video" is true to ensure
we download the submission as originally intended.
The rstrip function was used wrongly, it doesn't remove a substring but
rather removes any of the characters provided, so here it removed any I,
G, V, or F that finished the six character ID for Imgur, resulting in a
404 error for the resources in question.
