Enable runkit7 for the PHP 7.0 unit tests

[stevegrunwell]

Runkit7 dropped PHP 7.0 support in version 2.0.0, but releases < 2.0.2 are not available via PECL.

As a result, there isn't an easy way to instruct GitHub Actions to install runkit7-1.0.11 (the last of the 1.x branch) for the PHP 7.0 test. I also had a heck of a time getting this working over on stevegrunwell/runkit7-installer.

#20 broke the PHP 5.6–7.1 setups into separate steps, but 7.0 will currently fail to install runkit7 (meaning those tests will simply be skipped).

Eventually, it'd be nice to get this working for the sake of completion, but it's not worth holding the 0.2.0 release.