Update Client.php to replace depracated methods for newer PHP version

[Be-Klasen]

using iconv_set_encoding is deprecated in PHP>50600. The proposed change checks the PHP version and uses the new method instead when appropriate.

[jobbrown]

Why not version_compare() for this? http://php.net/manual/en/function.version-compare.php

Example:

if(version_compare(PHP_VERSION, '5.6.0') >= 0) {}