build: Adjust shell-version

With the new version scheme, only the major version is relevant as
far as gnome-shell is concerned. However the extension website does
not handle that at the moment, so always append a ".0".
5 jobs for master in 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Pre Review
passed check_commit_log #1216416

00:00:14

passed check-merge-request #1216417

00:00:20

 
  Review
passed eslint #1216419

00:00:21

passed js_check #1216418

00:00:13

 
  Build
passed build-bundles #1216420

00:00:29