feat(github): use GitHub actions for heroku app

This is to test using GitHub actions for building and deploying demo app to Heroku. Right now TravisCI is used for that purpose.

.github/heroku.workflow

@@ -0,0 +1,24 @@
+workflow "Deploy demo app to Heroku" {
+  on = "push"
+  resolves = "release"
+}
+
+action "login" {
+  uses = "actions/heroku@master"
+  args = "container:login"
+  secrets = ["HEROKU_API_KEY"]
+}
+
+action "push" {
+  uses = "actions/heroku@master"
+  needs = "login"
+  args = "container:push -a karma-demo web"
+  secrets = ["HEROKU_API_KEY"]
+}
+
+action "release" {
+  uses = "actions/heroku@master"
+  needs = "push"
+  args = "container:release -a karma-demo web"
+  secrets = ["HEROKU_API_KEY"]
+}