Travis CI use trusty instead precise (#3661)

It appears trusty has Yarn ^v1.0.0 now.

So (#3054) shouldn’t be an issue changing to trusty. As Travis CI [precise support](https://blog.travis-ci.com/2017-08-31-trusty-as-default-status) will be dropped in March 2018 .

.travis.yml

@@ -1,9 +1,5 @@
 ---
-# Use Ubuntu Precise instead of new default Trusty which cause build fail
-# with pre installed yarn v0.17.8
-# https://github.com/facebookincubator/create-react-app/issues/3054
-# TODO: remove after Trusty environment is updated with a lastet version of yarn
-dist: precise
+dist: trusty
 language: node_js
 node_js:
   - 6