If executing either `bin/test` in railties or `yarn build` in
actioncable, some build artifacts are left over. The master branch
git ignores those correctly.

However, checking out 5-2-stable means I'll suddenly have to deal with
them. Let's just ignore those artifacts here too.

Proof:

```
kaspth@kaspth-imac:~/code/rails (5-2-stable)$ git status
On branch 5-2-stable
Your branch is up to date with 'origin/5-2-stable'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)

        actioncable/app/javascript/
        railties/test/isolation/assets/
        tmp/
```