PHPUnit security vulnerability

[davidmpaz]

Hi,

lately I did start receiving some alert from Github about used version of PHPUnit and code injection. I know it is about a development dependency which should not be used in production, still I think it is good to have this sorted out, for the sake of peace of mind.

This PR does:

• Include Github Action workflow files for pipelines. Travis.org is no more since some time ago.
• Enforce usage of patched PHPUnit versions
• Include version 7.4 of php to the build matrix
• Test HHVM against latest version published with php support

[bobthecow]

Awesome, thank you so much!