Default to ensure => 'installed' for the package

Prior to this, the default value for the `ensure` parameter of the
logrotate package was set to 'latest'. This meant that by simply
declaring a logrotate::rule, the package could also be updated. It is
generally not a good idea to update system packages unless being done in
a controlled envrionment.

To fix that issue, the default value is being changed to 'intalled'.
This way, the end-user can choose to update the logrotate package when
they want to.

This fixes issue voxpupuli#11

manifests/init.pp

@@ -1,6 +1,6 @@
 #
 class logrotate (
-  $ensure            = 'latest',
+  $ensure            = 'installed',
   $hieramerge        = false,
   $manage_cron_daily = true,
   $package           = 'logrotate',