Downgrade circleci image to PHP 5.6

[sirbrillig]

This removes a null coalesce operator that was accidentally added in #83, creating a bug with older versions of PHP.

This also updates the circleci configuration to prevent such regressions from happening in the future.

Fixes #85

[GaryJones]

Why was the upgrade to PHP 5.6 needed?

WPCS and PHPCS require PHP >=5.4. I've also just made VIPCS support >=5.4 as well, and was hoping not to have to revert that when pulling in VA.

[sirbrillig]

Oh, good question. I doubt it was intentional but let me see. Looking at the things added by php 5.5 and php 5.6, the only thing that stands out to me is:

• Variadic functions using ..., but I could change that to func_get_args as it's just in one place.

I will create a branch that adds PHP 5.4 to the CI and see what happens.

[sirbrillig]

See #191 where I'll probably spend 5 hours trying to figure out how to get a php 5.4 environment (there's no pre-built docker image on circleci).