Re-enable the WordPress.Variables.GlobalVariables sniff

[jrfnl]

The WordPress.Variables.GlobalVariables sniff was originally part of the intended ruleset, but turned off because of its buggyness as reported in WordPress/WordPress-Coding-Standards#300.
See: https://github.com/Yoast/yoastcs/blob/master/Yoast/ruleset.xml#L48

All the currently known bugs are expected to be fixed in the near future once WordPress/WordPress-Coding-Standards#736 is merged.

It is expected that the PR will be part of the next WPCS release 0.11.0.

So I'd like to suggest re-enabling the sniff once WPCS 0.11.0 has been released and the minimum WPCS requirement in the Yoast CS composer.json has been upped to 0.11.0.

[jrfnl]

Update: the PR to fix the bugs in WPCS has been merged and will be part of the 0.11.0 release.

[jrfnl]

Update: WPCS 0.11.0 has been released.