Use more robust branch management

Require merge to 'master' prior to version bump and update 'last' after
release.

mkit.ini

     pkgname      =  saturnin-demo
     maintainer   =  Alois Mahdal <netvor+saturnin@vornet.cz>
     vcs_browser  =  https://github.com/AloisMahdal/saturnin-demo
-    relsrc       =  devel
+    relsrc       =  master
+    reldst       =  last
 
 [dist]
     tarball  = LICENSE.md