Increase maximum number of open files for HHVM from 1024 to 4096

HHVM on the job runner can hit the 1024 limit easily because it handles a large
volume of concurrent requests from the job dispatcher and because it makes its
own outbound requests to parsoid.svc.eqiad.wmnet, the databases, etc.

Bug: 70825
Change-Id: Ie1e1c0fede78d103cc00c12f03cfa03b16f4c6ba