[vimeo] Allow for iframe embeds that don't quote src.

Modern browsers are at least lenient enough to allow this and render the page
correctly so youtube_dl should be able to crawl these too.

youtube_dl/extractor/vimeo.py

@@ -523,7 +523,7 @@ class VimeoIE(VimeoBaseInfoExtractor):
         urls = []
         # Look for embedded (iframe) Vimeo player
         for mobj in re.finditer(
-                r'<iframe[^>]+?src=(["\'])(?P<url>(?:https?:)?//player\.vimeo\.com/video/\d+.*?)\1',
+                r'<iframe[^>]+?src=(["\']?)(?P<url>(?:https?:)?//player\.vimeo\.com/video/\d+.*?)\1',
                 webpage):
             urls.append(VimeoIE._smuggle_referrer(unescapeHTML(mobj.group('url')), url))
         PLAIN_EMBED_RE = (