Upgrade pathwatcher to prevent tests from hanging.

I fixed https://github.com/atom/node-pathwatcher/issues/47 with
https://github.com/atom/node-pathwatcher/pull/48, and @kevinsawicki
published my changes to NPM as pathwatcher@2.3.3.

These changes make it so pathwatcher no longer keeps our test processes
alive when they would otherwise naturally exit.