Stop leaking casperjs / phantomjs child process
Stop leaking casperjs / phantomjs child process

./full-run.sh used to fail with ENOMEM after spawning (and not reaping)
a bunch of 100MB+ child processes.

Auditors: csilvers

Test Plan:
Ran ./full-run.sh and then from another terminal checked the count of
phantomjs child processes:

  ps ax | grep phantomjs | wc -l

before the change, the number was ever-growing. Now, it holds steady.

Khan/exercise-icons,Khan/exercise-icons