Use is-reachable to detect child process health in self-test.

On Linux, child processes that have exited may remain as <defunct>
"zombie" processes, which prevents process.kill(childPid, 0) from
throwing, so we need a different trick for detecting whether the child
process is still alive.

scripts/dev-bundle-tool-package.js

@@ -41,6 +41,7 @@ var packageJson = {
     sqlite3: "3.1.8",
     netroute: "1.0.2",
     "http-proxy": "1.16.2",
+    "is-reachable": "3.1.0",
     "wordwrap": "0.0.2",
     "moment": "2.20.1",
     "rimraf": "2.6.2",