Update for Vimeo thumbnails

Vimeo seems to have changed their oembed thumbnail_url which broke the {video_mediumres} and {video_highres} tags. It also made {video_thumbnail} dimensions 200 x 150px. This patch brings them back in line with the YouTube thumbnail sizes.

{video_thumbnail}: 295px wide
{video_mediumres}: 640px wide
{video_highres}: 1280px wide