Do not require sudo on Travis CI (mastodon#6356)

The issue which the workaround for is now addressed:
travis-ci/travis-ci#7941 (comment)
> We've pushed out new stable trusty images to production with a patch.

.travis.yml

@@ -8,7 +8,7 @@ cache:
   - public/packs-test
   - tmp/cache/babel-loader
 dist: trusty
-sudo: required
+sudo: false
 branches:
   only:
   - master