Fix automatic coercion warning

Fix automatic coercion warning

Explicitly cast the return value of strftime() to an Integer to avoid
spamming the puppet server log file with warning messages:

Jan 26 14:19:13 <daemon.warn> marvin puppet-master[27147]: The string '1516972753' was automatically coerced to the numerical value 1516972753 at /usr/local/etc/puppet/environments/production/modules/apt/manifests/update.pp:17:27
Jan 26 14:19:25 <daemon.warn> marvin puppet-master[27147]: The string '1516972765' was automatically coerced to the numerical value 1516972765 at /usr/local/etc/puppet/environments/production/modules/apt/manifests/update.pp:17:27
Jan 26 14:19:55 <daemon.warn> marvin puppet-master[27147]: The string '1516972795' was automatically coerced to the numerical value 1516972795 at /usr/local/etc/puppet/environments/production/modules/apt/manifests/update.pp:17:27
Jan 26 14:20:15 <daemon.warn> marvin puppet-master[27147]: The string '1516972815' was automatically coerced to the numerical value 1516972815 at /usr/local/etc/puppet/environments/production/modules/apt/manifests/update.pp:17:27