HHVM: set MALLOC_CONF envvar in upstart job def

Have HHVM start with jemalloc's memory profiling capabilities enabled but
inactive. Profiling can then be toggled on-the-fly by cURLing the
/jemalloc-prof-* endpoints of the admin server.

Change-Id: I74c8f3376a7e1e495ea68815bf36a0b1cbc4c7c2