feat: Update Ubuntu 18.04 to build latest PHP 7.2
feat: Update Ubuntu 18.04 to build latest PHP 7.2

Ubuntu 18.04 installs PHP 7.2 by default via its APT packages.

Now this cookbook will also build the latest PHP 7.2 from source, to
match the packages.

opscode-cookbooks/php,chef-cookbooks/php,opscode-cookbooks/php,chef-cookbooks/php