Fix missing php7-simplexml plugin

Fix missing php7-simplexml plugin

This is to fix this error I got:

  Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for slim/slim 3.12.3 -> satisfiable by slim/slim[3.12.3].
    - slim/slim 3.12.3 requires ext-simplexml * -> the requested PHP extension simplexml is missing from your system.

  To enable extensions, verify that they are enabled in your .ini files:
    - /etc/php7/php.ini
    - /etc/php7/conf.d/00_curl.ini
    - /etc/php7/conf.d/00_iconv.ini
    - /etc/php7/conf.d/00_json.ini
    - /etc/php7/conf.d/00_mbstring.ini
    - /etc/php7/conf.d/00_openssl.ini
    - /etc/php7/conf.d/01_phar.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.