Revert "Set travis dist to "precise"" (#3510)

This reverts commit 675688c814.

See the following for context:
* https://github.com/prettier/prettier/pull/3349#issuecomment-350617020
* https://github.com/travis-ci/travis-ci/issues/7566#issuecomment-351514693

.travis.yml

@@ -3,7 +3,6 @@ language: node_js
 node_js:
   - 4
   - stable
-dist: precise
 cache:
   yarn: true
   directories: