5.x.x: Update validate-json to use spl_autoload_register

[sanmai]

Fixes #574

• spl_autoload_register exists since PHP 5.1, so this shouldn't be a problem backward-compatibility wise.
• There's was a more drastic change ([BUGFIX] Fix autoload to work properly with composer dependencies #401 & 9fe7822) in the master to fix the same root issue, but it wasn't backported to 5.x. Hence the need for this fix.

[erayd]

I'm ok with this one merging directly into 5.x.x, as the relevant code has diverged from v6.

[erayd]

Thank you @sanmai :-).

[maks-rafalko]

Guys, could you please tag a new release with this fix?

[erayd]

@maks-rafalko Yes. I want to roll a few other backported fixes in too.

[sanmai]

A release with just this fix will be very appreciated because it is a major deprecation since PHP 7.2. Almost everyone these days is getting PHP 7.3 from the start, e.g. because Debian stable includes one. Thank you very much for considering this request.

[erayd]

@sanmai Are you requesting a release ASAP, or are you specifically wanting a release with that fix only in it? Because I'm doing the backports now, and should be tagging stuff in a few minutes.

[erayd]

@maks-rafalko I've just tagged v5.2.9 with all current backports merged in.

@sanmai Please let me know if you need an additional release that contains only the autoload fix - that seems like a rather strange thing to ask for, so I'd prefer not to do it without a good reason.

[maks-rafalko]

Thank you!

[sanmai]

Please let me know if you need an additional release that contains only the autoload fix

This wasn't necessary absolutely. I meant that if you're busy with other things tagging a release with just this fix would be a big help. But else any release works. Thank you again.