Update Travis to use precise
Update Travis to use precise

Newer versions like trusty don't have PHP 5.2 or 5.3

https://blog.travis-ci.com/2017-07-11-trusty-as-default-linux-is-coming

GoogleForCreators/web-stories-wp,GoogleForCreators/web-stories-wp,GoogleForCreators/web-stories-wp,GoogleForCreators/web-stories-wp,GoogleForCreators/web-stories-wp