Fix .travis.yml assertions

The underlying blueprint was updated in the `beta` branch to use
`xenial` instead of `trusty`, then those changes were merged into
`master` branch. However, `master` branch had made some small changes to
these tests as well (adding a test for `ember new` without special
flags) that needed fixing also.

tests/fixtures/app/defaults/.travis.yml

@@ -3,7 +3,7 @@ language: node_js
 node_js:
   - "10"
 
-dist: trusty
+dist: xenial
 
 addons:
   chrome: stable