avformat/dashenc: change the hls version from 6 to 7

reference hls support fmp4 file from  draft-pantos-http-live-streaming-20
the spec describes version 7 of hls protocol

Suggested-by: Ronak <ronak2121@yahoo.com>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavformat/dashenc.c

@@ -827,7 +827,7 @@ static int write_manifest(AVFormatContext *s, int final)
         }
         av_dict_free(&opts);
 
-        ff_hls_write_playlist_version(out, 6);
+        ff_hls_write_playlist_version(out, 7);
 
         for (i = 0; i < s->nb_streams; i++) {
             char playlist_file[64];