Remove PHP v5.x support and add basic doc for how one may install the old version

src/Curl/Decoder.php

@@ -16,14 +16,6 @@ class Decoder
     public static function decodeJson()
     {
         $args = func_get_args();
-
-        // Call json_decode() without the $options parameter in PHP
-        // versions less than 5.4.0 as the $options parameter was added in
-        // PHP version 5.4.0.
-        if (version_compare(PHP_VERSION, '5.4.0', '<')) {
-            $args = array_slice($args, 0, 3);
-        }
-
         $response = call_user_func_array('json_decode', $args);
         if ($response === null) {
             $response = $args['0'];