Fix issue #176 - PHP5.2 compatibility

As reported in issue #176

> Found when running a travis build: https://travis-ci.org/Yoast/wordpress-seo/builds/28375064

> Considering the minimum requirement for WP is still (unfortunately) PHP 5.2.6, I'd suggest that the codesniffs should also be able to work under PHP 5.2.

> Parse error: syntax error, unexpected T_FUNCTION in /home/travis/.phpenv/versions/5.2.17/pear/PHP/CodeSniffer/Standards/WordPress/Sniffs/Variables/VariableRestrictionsSniff.php on line 140

I'm a bit allergic to functions (auto-)passing variables by reference as it often makes code harder to decipher. `array_map` is a nice alternative in this case ;-) Glad you approve.