Fix frontend tests on Travis (#8089)

* Remove obsolete travis cache option (apt)

See: https://github.com/travis-ci/travis-ci/issues/5876

* Travis config value "addons.postresql" is a string

* Update Travis from trusty to xenial

Fixes the frontend build, which was failing because Chrome has dropped Trusty support in the latest version. See: https://travis-ci.community/t/travis-is-downloading-older-chrome-versions-in-place-of-chrome-stable/5040/4

.travis.yml

@@ -21,7 +21,7 @@ env:
 
 addons:
   chrome: stable
-  postgresql: 9.6
+  postgresql: "9.6"
   apt:
     update: true
     packages:
@@ -40,10 +40,9 @@ services:
   - redis-server
 
 sudo: required
-dist: trusty
+dist: xenial
 
 cache:
-  apt: true
   yarn: true
   directories:
     - vendor/bundle