Add php-dom extension for CentOs platform.

The build fails on my CentoOs6.3 base box because drush install throws
an Exception when it does not find the php-dom extension.

The minimum PHP package on CentOs does not include the php-dom extension by default.

Note there is no php:module_dom recipe to add via the requirement stament.  Also, the
maintainer of the php cookbook has depricated all the module_<extension>
recipes, and is asking developers that require these extensions to include them via a call to package as I do here.
Require cron recipe in default recipe.

Some small CentOs installations do not come with cron by default. This
change helps ensure that a build does not fail because crontab is not
available.
Merge pull request #10 from jenkinslaw/add-php-dom

Add php-dom extension for CentOs platform.
Merge pull request #11 from jenkinslaw/add-cron-recipe

Require cron recipe in default recipe.
make foodcritic happy again; move the cron include in the cron recipe; use platform_family instead of platform to catch more platforms here
Check for a default site before disabling it.

The build fails on our centos63 platform because the default site is not
enabled and the `a2dissite` command throws an error when it does not find it.
This commit corrects this issue by making sure that command is only envoked
when a default site is present.

Note that in the current apache2 cookbook `default['apache']['default_site_enabled']`
is set to `false` for all platforms.
Merge pull request #12 from jenkinslaw/check-default-site

Check for a default site before disabling it.