Wp-cli: Travis testing of PHP 5.3 code

[schlessera]

It seems that, starting september 2017, we will not be able to test the PHP 5.3 matrix in Travis anymore.

They will switch the default Ubuntu distro from precise to trusty.

Related issue: https://github.com/travis-ci/travis-ci/issues/8072

We need to plan our next steps for dealing with this. This basically leaves us with three options:

A. Switch to docker-based testing to allow us to use our own images.
B. Test from PHP 5.4+ and hope for the best.
C. Drop official PHP 5.3 support.

[schlessera]

From what I've been able to read in the change logs and the documentation, fthe sudo: required container might still come with PHP 5.3.29.

So, the combination of sudo: required and dist: precise might still do the trick _after_ September 1st, 2017. We'll have to run tests, though, as I've read about conflicting results with this approach.

[danielbachhuber]

C. Drop official PHP 5.3 support.

I'd be amenable to this.

[danielbachhuber]

It seems that, starting september 2017, we will not be able to test the PHP 5.3 matrix in Travis anymore.

According to this blog post, we have until April 1st, 2018.

[danielbachhuber]

We'll bump minimum to PHP 5.4 at the beginning of February #4554