Change the query, and the trigger before purge (1.5 time the max row count will trigger the purge to keep only max row count records).

This should work on any SQLite3 binaries.
Merge branch 'Fix-the-Purge-query-for-Hypervisor-statistique-(no-more-growing)' of https://github.com/CyrilPeponnet/Archipel into pr-624
This query should work *even* on some old centos 6 distro !

it removes 0.5 of the maximum row (the oldest) when the database
reaching 1.5 times the max row settings.

Thanks to Primalmotion to have tried on his oldddddd centos 6 :p
Merge branch 'Fix-the-Purge-query-for-Hypervisor-statistique-(no-more-growing)' of https://github.com/CyrilPeponnet/Archipel into pr-624