[arte] "quality" key test

When the "quality" key does not exist, it crashes the application.
So the existence of "quality" key must be tested before requesting it.
You can test with this command, with the current version youtube-dl crashes
youtube-dl -F http://www.arte.tv/guide/fr/052414-003/repas-de-fete

youtube_dl/extractor/arte.py

@@ -106,6 +106,7 @@ class ArteTVPlus7IE(InfoExtractor):
         formats = []
         for format_id, format_dict in player_info['VSR'].items():
             f = dict(format_dict)
+            if  'quality' in f :
                 versionCode = f.get('versionCode')
     
                 langcode = {