HHVM: increase JitAColdSize to 60 MiB

We had one crash in the past week caused by lack of free space in the cold JIT
cache.

The default values for the frozen and cold caches are 40 and 24 MiB,
respectively. Since we set the max size of the frozen cache to 100 MiB, we can
set the max for the cold cache at 60 MiB to keep the same ratio.

Change-Id: I9cd085baca836da969c0882a57b0022b02a5f7a0