Add emacs-28 in test matrix

Author: ikappaki

.circleci/config.yml

@@ -33,6 +33,12 @@ jobs:
         entrypoint: bash
     <<: *default-steps
 
+  test-emacs-28:
+    docker:
+      - image: silex/emacs:28-ci-cask
+        entrypoint: bash
+    <<: *default-steps
+
   test-emacs-master:
     docker:
       - image: silex/emacs:master-ci-cask
@@ -46,4 +52,5 @@ workflows:
       - test-emacs-25
       - test-emacs-26
       - test-emacs-27
+      - test-emacs-28
       - test-emacs-master