Update snapcraft.yaml to new version

Author: Siedlerchr

snap/snapcraft.yaml

@@ -58,7 +58,7 @@ environment:
 parts:
   jabref:
     plugin: dump
-    source: https://builds.jabref.org/main/JabRef-5.14-portable_linux.tar.gz
+    source: https://builds.jabref.org/main/JabRef-5.15-portable_linux.tar.gz
     stage-packages:
       - x11-utils
     override-build: |