Issue #2048: fix preg_replace deprecation warning

The linked issue has to do with deprecation warnings showing up when running the master branch against PHP 5.5. This is because 5.5 has [deprecated a PCRE pattern](http://php.net/manual/en/function.preg-replace.php), and Smarty 2.6.26 uses that pattern in conjunction with preg_replace.

Updating to Smarty's 2.6.28 release makes the library to be PHP 5.5 safe, and also includes some security fixes.