I have a Jenkins job which runs unit tests on different PHP versions. To do this, I have a `PHP_VERSION` axes. I would like to ensure that our unit tests are passing on PHP 5.3, 5.4, 5.5 and 5.6 but not on PHP 7. That is, I would like to run our unit test suite on PHP 7, but I do not care if the job fails.

It doesn't seem possible to create an optional variant in a matrix axes.