travis: install PHP 5.2 and 5.3 as needed

[JDGrimes]

See https://core.trac.wordpress.org/ticket/41292

[JDGrimes]

Looks like this hasn't actually been addressed in core yet, but it has been fixed in Gutenberg: WordPress/gutenberg@3c34274

[JDGrimes]

It looks like we can still gain a very little time by using dist: precise, although that was planned to be removed this month. See travis-ci/travis-ci#8072.

Maybe that would allow us to wait until the solution was committed to WordPress core, so we'd be able to bootstrap it instead of copying it over.

[JDGrimes]

Here is the timeline: https://blog.travis-ci.com/2017-08-31-trusty-as-default-status

Short story, we'll loose support for sudo: false with dist: precise sometime between now and the 22nd, which will make it necessary to install PHP 5.2 and 5.3 manually.