AttributeError: 'InProcessKernel' object has no attribute 'io_loop'

I filed a similar bug under the `jupyter/qtconsole` project, but later narrowed the problem down to `ipykernel`, so I'm opening a new bug report here.

I was working with an IPython qtconsole embedded in some C++ code, which used to work. But after a recent update, I cannot exit the console/quit my app using exit/exit()/quit/quit().

The problem seems to have been introduced in `ipykernel==4.7`, downgrading to `ipykernel==4.6.1` solves the problem. Here's a minimal test example:
```
import os
os.environ['QT_API'] = 'pyside'
from qtconsole.qt import QtGui
from qtconsole.rich_jupyter_widget import RichJupyterWidget
from qtconsole.inprocess import QtInProcessKernelManager
from IPython.lib import guisupport

class QIPythonWidget(RichJupyterWidget):

def __init__(self, *args, **kwargs):
super(RichJupyterWidget, self).__init__(*args, **kwargs)
self.kernel_manager = QtInProcessKernelManager()
self.kernel_manager.start_kernel()
self.kernel = self.kernel_manager.kernel
self.kernel.gui = 'qt4'
self.kernel.shell.push(kwargs)
self.kernel_client = self.kernel_manager.client()
self.kernel_client.start_channels()

def stop():
self.kernel_client.stop_channels()
self.kernel_manager.shutdown_kernel()
guisupport.get_app_qt4().exit()
self.exit_requested.connect(stop)

if __name__ == "__main__":
app = QtGui.QApplication([])
wid = QIPythonWidget()
wid.show()
app.exec_()
```

The error returned is the following:
```
In [1]: exit
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
in ()
----> 1 exit()

/usr/local/lib/python2.7/dist-packages/IPython/core/autocall.pyc in __call__(self, keep_kernel)
68 def __call__(self, keep_kernel=False):
69 self._ip.keepkernel_on_exit = keep_kernel
---> 70 self._ip.ask_exit()

/usr/local/lib/python2.7/dist-packages/ipykernel/zmqshell.pyc in ask_exit(self)
526 def ask_exit(self):
527 """Engage the exit actions."""
--> 528 self.exit_now = (not self.keepkernel_on_exit)
529 payload = dict(
530 source='ask_exit',

/usr/local/lib/python2.7/dist-packages/traitlets/traitlets.pyc in __set__(self, obj, value)
583 raise TraitError('The "%s" trait is read-only.' % self.name)
584 else:
--> 585 self.set(obj, value)
586
587 def _validate(self, obj, value):

/usr/local/lib/python2.7/dist-packages/traitlets/traitlets.pyc in set(self, obj, value)
572 # we explicitly compare silent to True just in case the equality
573 # comparison above returns something other than True/False
--> 574 obj._notify_trait(self.name, old_value, new_value)
575
576 def __set__(self, obj, value):

/usr/local/lib/python2.7/dist-packages/traitlets/traitlets.pyc in _notify_trait(self, name, old_value, new_value)
1137 new=new_value,
1138 owner=self,
-> 1139 type='change',
1140 ))
1141

/usr/local/lib/python2.7/dist-packages/traitlets/traitlets.pyc in notify_change(self, change)
1174 c = getattr(self, c.name)
1175
-> 1176 c(change)
1177
1178 def _add_notifiers(self, handler, name, type):

/usr/local/lib/python2.7/dist-packages/ipykernel/zmqshell.pyc in _update_exit_now(self, change)
470 """stop eventloop when exit_now fires"""
471 if change['new']:
--> 472 loop = self.kernel.io_loop
473 loop.call_later(0.1, loop.stop)
474 if self.kernel.eventloop:

AttributeError: 'InProcessKernel' object has no attribute 'io_loop'

In [2]:
```
The output of `python -c "import IPython; print(IPython.sys_info())"` on my machine is:
```
{'commit_hash': u'b467d487e',
'commit_source': 'installation',
'default_encoding': 'UTF-8',
'ipython_path': '/usr/local/lib/python2.7/dist-packages/IPython',
'ipython_version': '5.5.0',
'os_name': 'posix',
'platform': 'Linux-4.10.0-42-generic-x86_64-with-LinuxMint-18.3-sylvia',
'sys_executable': '/usr/bin/python',
'sys_platform': 'linux2',
'sys_version': '2.7.12 (default, Dec 4 2017, 14:50:18) \n[GCC 5.4.0 20160609]'}
```

The pip versions of the (relevant) packages on my system are:
```
ipykernel==4.8.2
ipython==5.5.0
ipython-genutils==0.2.0
ipywidgets==7.1.2
...
jupyter==1.0.0
jupyter-client==5.2.2
jupyter-console==5.2.0
jupyter-core==4.4.0
...
pyzmq==17.0.0
qtconsole==4.3.1
tornado==5.0
traitlets==4.3.2
```
And I'm using the system package for PySide, version 1.2.2