Changelog: replace next by 2.0

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Changelog

 Entries are sorted chronologically from oldest to youngest within each release,
 releases are sorted from youngest to oldest.
 
-version <next>:
+version 2.0:
+
 - curves filter
 - reference-counting for AVFrame and AVPacket data
 - ffmpeg now fails when input options are used for output file