Distributor: Add WP Acceptance tests for PHP 5.6 and WordPress 4.7

[jeffpaul]

Is your enhancement related to a problem? Please describe.
Our requirements state that you need at least PHP 5.6 and WordPress 4.7 to run Distributor. So let's make sure we've got a WPA test that covers those.

Describe the solution you'd like

• [ ] add WPA test to cover PHP 5.6 and WP 4.7

Designs
n/a

Describe alternatives you've considered
none

Additional context
none

[adrianduffell]

I can work on a PR for this.

[jeffpaul]

Note that @ryanwelcher has a draft PR started on this here: #537