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

src/Curl/Encoder.php

@@ -20,14 +20,6 @@ class Encoder
     public static function encodeJson()
     {
         $args = func_get_args();
-
-        // Call json_encode() without the $depth parameter in PHP
-        // versions less than 5.5.0 as the $depth parameter was added in
-        // PHP version 5.5.0.
-        if (version_compare(PHP_VERSION, '5.5.0', '<')) {
-            $args = array_slice($args, 0, 2);
-        }
-
         $value = call_user_func_array('json_encode', $args);
         if (json_last_error() !== JSON_ERROR_NONE) {
             if (function_exists('json_last_error_msg')) {