Load the cron cookbook.

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.

This is related to drupal-cookbook issue promet/drupal-cookbook#11

Cheffile

@@ -12,6 +12,7 @@ cookbook 'apache2', '>= 1.1.0'
 cookbook 'php', '>=1.0.0'
 cookbook 'percona-install'
 cookbook 'memcached', '>=1.0.0'
+cookbook 'cron'
 cookbook 'drupal',
   :git => 'https://github.com/mdxp/drupal-cookbook.git'