Update php-cs-fixer config to specify native_constant_invocation

[michaeljoseph]

Maybe update the php-cs-fixer config in a separate change?
https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/2.18/doc/rules/function_notation/native_function_invocation.rst

Originally posted by @michaeljoseph in #20 (comment)