Add "current", "next" and "oldest" version as an alias to different PHP versions

Currently there is 7.4 to be released in few days. I used to use “nightly” but it’s now PHP8.

Generally for libraries, it would be useful to have an option to test with (as per as of 2019-11-15):

  • oldest (7.1)
  • current (7.3)
  • next (7.4snapshot)

Motivation for this is that I want to make sure the library works on those versions all the time. And it’s a lot of useless work to update the list every time PHP version is released.

What do you think?