Running puppet apply every 30mins

- running puppet every 30 mins on mirror boxes.Introduced a generic class cron which takes a command and configures it to run every 30 mins.

- removed support for puppet 2.7 in Travis because of  issue http://projects.puppetlabs.com/issues/8814. This issue is fixed in 3.0. This module is currently consumed only by mirror which uses puppet 3.0.