chore: Fix failed build

- you must use the flag --allow-unauthenticated when running apt install
  with unauthenticated packages. Since we are not installing it in a
Dockerfile, I'm not sure how to pass this flag in our travis.yml.
Looking at the docs, it's not clear why we were installing it with apt
though since they recommend using an addon. https://docs.travis-ci.com/user/chrome#selecting-a-chrome-version

.travis.yml

@@ -9,11 +9,6 @@ addons:
   ssh_known_hosts: rundfunk-mitbestimmen.de
   postgresql: "9.6"
   chrome: stable
-  apt:
-    sources:
-      - google-chrome
-    packages:
-      - google-chrome-stable
 
 before_install:
   - . $HOME/.nvm/nvm.sh