bfix: shutdown_server returns True when pid exists

`check_pid` returns `True` if the PID for a notebook server still exists. Therefore, the `if check_pid(pid):` statements on lines 424 and 437 evaluate to `True` even though the notebook server is still running.

This commit simply adds a `not` to each line: `if not check_pid(pid):` so that the conditional only evaluates to `True` if `check_pid` returns `False`, which happens when the notebook server has shutdown, as expected.

notebook/notebookapp.py

@@ -421,7 +421,7 @@ def shutdown_server(server_info, timeout=5, log=None):
 
     # Poll to see if it shut down.
     for _ in range(timeout*10):
-        if check_pid(pid):
+        if not check_pid(pid):
             if log: log.debug("Server PID %s is gone", pid)
             return True
         time.sleep(0.1)
@@ -434,7 +434,7 @@ def shutdown_server(server_info, timeout=5, log=None):
 
     # Poll to see if it shut down.
     for _ in range(timeout * 10):
-        if check_pid(pid):
+        if not check_pid(pid):
             if log: log.debug("Server PID %s is gone", pid)
             return True
         time.sleep(0.1)