Bump PHP memory limit from 200M to 800M in populate_adm_tree.php

The PHP script populate_adm_tree.php was failing with

2018-07-24_13:13:47 Update admin structure tree...
Deleting previous data...
Rebuilding table...

PHP Fatal error:
Allowed memory size of 209715200 bytes exhausted (tried to allocate 32 bytes)
in erfgoedbot/populate_adm_tree.php on line 45

Increasing the memory limit is the obvious solution.
Testing on Toolforge, 500M was not enough, 800M was.

Bug: T200337
Change-Id: I32d9d806f6a440e42cfed6d4775d47ec3cf94df2