make compatible with twig 3.9, twig_get_attribute is now internal func

[jtallant]

Merging this allows the package to run on Twig version 3.9. twig_get_attribute was moved to an internal function by twig so we can no longer directly call it.

Fixes #440

[rudiedirkx]

GetAttrNode::attribute() is executed A LOT on every page. Is repeating function_exists() fast enough? We only have to do that once? I can imagine PHP has to do some lookup everytime it's called.

[jtallant]

Not sure about that. I suppose we can just not check if the function exists and just return the call to Core. Either way this package doesn't work with 3.9 until this is merged.

[barryvdh]

Can we just bump the Twig requirement and skip the check? Not sure which version includes it, but if we use that version, we can just remove the old function.

[willrowe]

This just bit me pretty bad and caused a crash in production. I was on an older version of this package and updated twig/twig due to a security vulnerability. Since the version constraint for twig/twig on the older version allowed all minor versions of v3, it updated without any kind of warning.

I'm not sure if this is even possible, but can previous versions be constrained to only versions before v3.9 of twig.twig?