ValidVariableNameSniff.php raises Deprecation Error in PHP 5.5

In Sniffs/NamingConventions/ValidVariableNameSniff.php there is
a preg_replace with switch /e as callback.

In PHP 5.5 this is deprecated and should get replaced with
preg_replace_callback. The following change fixes this.

Resolves: #15
Release: 1.0.0
Stick on the CGL of TYPO3 and change null to NULL