List required ext-xml in composer.json.

[mbaynton]

I think it's easy enough for a user's first experience with terminus to go something like:

$ curl -O https://raw.githubusercontent.com/pantheon-systems/terminus-installer/master/builds/installer.phar && php installer.phar install
$ bin/terminus
PHP Fatal error:  Uncaught ReflectionException: Class DOMDocument does not exist in /home/mpantheon/terminus/vendor/consolidation/output-formatters/src/Transformations/DomToArraySimplifier.php:24

This PR is one of several possible UX improvements, with just this we get at installation

Loading composer repositories with package information
Installing dependencies from lock file
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - The requested PHP extension ext-xml * is missing from your system. Install or enable PHP's xml extension.

We could perhaps also iterate on this to add our own checks in the installer and/or in terminus itself for users of the .phar, but I think this change is fine regardless to make the composer.json more correct.