Leaking phantomjs processes

Created by: mossharris

description: OS： Ubuntu 16.04.1 LTS 64 bit Configuration: 8 nuclear 16 G

You're using 3.35g memory, created 5 tasks by web-UI interface (./bin/arachni_rpcd) and waiting for the ends of scaning. the current system memory is 4.86G and more processes at the same time（arachni/lib/arachni/processes/executables/base.rb）