Switch from proc-wait3 to rusage gem.

proc-wait3 monkey patches `$?` which was causing
problems for poltergeist. rusage is lighter weight
and provides the bit of proc-wait3 we care about.
usage is different on OS X and Linux, apparently.

On my computer, it returns the memory usage in bytes.
On a linux server, I SSH'd into, it returns it in KB.

The OS X (BSD) getrusage man page says it is in bytes.
The Linux man page says it is in KB as of a particular
Linux version.

We deal with this by comparing the `ps` approach to
what rusage returns, and optionally dividing by 1024
if it appears that the amount is coming back in raw bytes.

We also switched everything to KB while we are at it
as that is easier to deal with.
Harden the test against flickering.

I found it was passing in isolation but failing
when run with the whole suite, depending on when
it runs.  It's best to make it relative to the
current memory usage of the process.