Update for Chef 15 license agreement and Chef Workstation

.travis.yml

-sudo: required
-dist: trusty
-
 addons:
   apt:
     sources:
-      - chef-current-trusty
+      - chef-current-xenial
     packages:
-      - chefdk
+      - chef-workstation
 
 # Don't `bundle install` which takes about 1.5 mins
 install: echo "skip bundle install"
 
+env:
+  - CHEF_LICENSE=accept
+
 branches:
   only:
     - master