Wrong cache expiration value when sc_simple_cache option was set before 1.6.4

[benoitchantre]

With #62, the unit of cache expiration was changed from minutes to hours.
The default value for cache expiration is now 24 hours instead of 1440 minutes. The result is the same, but easier to read.

This change works for new installations of Simple Cache, but for existing ones, the numeric value is kept and the unit is set to hours instead of minutes. If the administrator doesn't notice the change and save Simple Cache configuration, the cache expiration value will be multiplied by 60.

I think the simplest and safest way to fix that is to set back 1440 minutes as default.