fixes #6237 Reading php_uname in case shell_exec failed.

According to http://forum.piwik.org/read.php?2,113613,page=1#msg-113739
php_uname will not return full string as in the shell, so we must first read shell_exec output

core/CliMulti/Process.php

@@ -168,6 +168,11 @@ class Process
     private static function isSystemNotSupported()
     {
         $uname = @shell_exec('uname -a');
+
+        if(empty($uname)) {
+            $uname = php_uname();
+        }
+
         if(strpos($uname, 'synology') !== false) {
             return true;
         }