Update travis config to use new infrastructure

[mboynes]

Per https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures, the container-based infrastructure (triggered by sudo: false) is now deprecated. The Travis config should be updated accordingly. This might cause other issues related to phpunit versioning, so after updating, a scaffolded plugin should be tested using Travis.

[schlessera]

The main issue we're facing with this is that we'll also have to update the PHP minimum requirement for the tests to 5.4+. When I last tested, the new infrastructure didn't work properly on PHP 5.3.