Yup, that error you saw is as I said, "the new shorthand array syntax introduced in 5.4", which allows you to specify arrays as ...
$foo = ['one', 'two', 'three'];
... instead of the original 5.3 way of ...
$foo = array('one', 'two','three');
And indeed guzzle uses the new syntax. And as I said, "while we avoid using it, to try and stay within J!'s 5.3.10 minimum version, we now ship a number of 3rd party libs maintained via the Composer dependency manager, which may use that syntax."
And although the stuff in the Composer library isn't used by the "core" of Fabrik, it is used by our installable plugins. And the problem is, the way Composer works, we can't have a separate Composer library for each plugin. So for example, the Stripe payment gateway form plugin uses guzzle (as a dependency on the stripe API composer lib). And although you may not be using Stripe, and don't have it installed, the only way we can manage and distribute that library is as part of the monolithic ./libraries/fabrik composer lib (now an installable lib_fabrik.zip).
The only way we could have separate Composer libs would be if I split out every single plugin into it's own github repo, with it's own IDE project and build file. Which would be a complete nightmare.
So moving forward, Fabrik's minimum requirement is PHP 5.4. Although frankly, I only develop and test against 7.1, and would strongly recommend that everyone running a J! / Fabrik site runs 7.1.
-- hugh