Bump ulimit max size for corefiles

The corefile generated just now for the PHPUnit segfault says
BFD: Warning: /home/hashar/bug62623.core is truncated: expected core
file size >= 651972608, found: 65609728.
...so, clearly, 64MB was overoptimistic. Bump this to an arbitrary 2GB
limit, but this means to be way more aggressive on our clean ups (e.g. a
day, instead of a week).

Change-Id: Ic4a6397ab8bb1592d0346ea04be4f756651dc84e