Flush stdout and stderr prior to exiting child process.

This fixes a confusing bug that we ran into where the output from
certain child processes wouldn't appear when running `lint-all` via
`ssh` (without a tty) into the Vagrant environment.

tools/lint-all

@@ -373,6 +373,8 @@ def run_parallel():
             logging.info("start " + name)
             result = func()
             logging.info("finish " + name)
+            sys.stdout.flush()
+            sys.stderr.flush()
             os._exit(result)
         pids.append(pid)
     failed = False