Improve zombie_killer compatability

[grantwest]

Problem:

• Combination of zombie_killer and test command quotes are causing
  the unit tests to not run

Solution:

• Remove use of exec in sh script because it is being used incorrectly
• Change command quoting to match what is conventionally used with sh -c

Fixes #110

[rjdellecese]

When I run this version of the script on MacOS it works (the tests run), but it re-raises the issue that #106 was intended to address—that is, it doesn't kill the test process when the watcher is killed. I can see this happen when I run mix test.watch on a test suite that runs for about 20s, killing the watcher with Ctrl-C, and then running ps -ef | grep mix.

I don't think I understand the issue arising from using read with no args (though I'm no shell scripting expert), and I wasn't able to reproduce it on my version of sh on my machine, but when I run the script with only the while read UNUSED; do :; done change, it still works for me. Does that solve the problem you were running into?

Another possibility would just be replacing #!/usr/sh with #!/usr/bash—I know that technically sh is more portable than bash, but what *nix system doesn't have bash? Maybe bash would still be portable enough?