update getFreeMemory when it's called very two seconds

getFreeMemory is called every two seconds when checking to see if the system is low on memory. Before this commit, half of these checks are useless (when okular is idle) since the cache is used when the cache is <=2 seconds old. This means that after the system is out of memory, up to 4 seconds (instead of 2) could go by before okular starts to free memory.