Getting PHP warning in ee shell with php 5.6

I created a site with following command:

```
ee site create example.com --type=wp --mu=subdir --php=5.6 --cache
```

and in fresh site, i can see following working in `ee shell`.

```
^_^[www-data@example.com:~/htdocs]$ php -v
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20131226/memcached.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20131226/memcached.so: cannot open shared object file: No such file or directory in Unknown on line 0
PHP 5.6.38 (cli) (built: Nov 16 2018 07:27:46)
Copyright (c) 1997-2016 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2016 Zend Technologies
with Zend OPcache v7.0.6-dev, Copyright (c) 1999-2016, by Zend Technologies
```