Switch to master branch to get it to compile on the Snap farm

Signed-off-by: Olivier Paroz (oparoz) <github@oparoz.com>

snapcraft.yaml

@@ -1,5 +1,5 @@
 name: spreedme
-version: 0.28.1snap1
+version: 0.29-dev.snap1
 summary: Spreed.ME audio/video calls and conferences feature for the Nextcloud Snap
 description: Spreed.ME implements a WebRTC audio/video call and conferencing server and web client which is available directly inside Nextcloud
 grade: stable
@@ -17,7 +17,7 @@ parts:
     plugin: autotools
     source: https://github.com/strukturag/spreed-webrtc.git
     source-type: git
-    source-branch: release-0.28
+    source-branch: master
     build-packages: [golang-go, nodejs]
 
   # Spreed.ME Nextcloud app