HHVM: update JIT settings

* Update configuration key names for <https://github.com/facebook/hhvm/commit/ca99ef1>.
* Keep the ratio of a_size, a_cold, and a_frozen to have a ratio of 1 : 0.33 : 1, per
  Brett's advice.

Change-Id: I192efa726a76bcc799490a164cf9bed0e56d5552