Bump minimum version to PHP 7.1

[MauricioFauth]

Drop support for PHP 5.3, PHP 5.4, PHP 5.5, PHP 5.6, PHP 7.0 and HHVM

Since phpMyAdmin requires PHP 7.1, it makes sense to require PHP 7.1 for sql-parser as well.

[remicollet]

I think a v4 branch is needed... current phpMyAdmin (4.8.x) still rely on it

[ibennetch]

Anyone who still uses the version 4 can pull it from the Git tag v4.3.1 or GitHub release 4.3.1, minimizing the importance of having the branch — but I think that creating the branch now will encourage fixes to be backported to v4 by authors, so I do think it's a good idea and will take care of it.

[ibennetch]

We should decide on a name for the branch, though; I'm partial to v4 or version4...keeping with the phpMyAdmin style we might also prefer QA_4_3. Any opinions?

[MauricioFauth]

@ibennetch What about 4.x? I also like v4.

[ibennetch]

Both of those are good ideas. Since no one has really offered any other suggestion I'm thinking 4.x is probably best. I'll make the branch from the release tag sometime this week.