Fix crash when a new MR was created

managetests/app.py

@@ -157,7 +157,7 @@ class TestsManager:
 
         # Send an alert on the merge request, if this branch has one
         if mr is not None:
-            self.gitlab.post_command(mr, "L'[istanza live]"
+            self.gitlab.post_comment(mr, "L'[istanza live]"
                                          "(http://wwwtest.ubuntu-it.org/%s/) "
                                          "per il branch **%s** è stata "
                                          "aggiornata." % (branch, branch))