remove the only explicite io_loop argument

This requires Tornado>=4.1.

nvchecker/source/cmd.py

@@ -23,7 +23,7 @@ def _run_command():
     cmd_q.running = False
     return
 
-  p = tornado.process.Subprocess(cmd, shell=True, io_loop=IOLoop.instance(),
+  p = tornado.process.Subprocess(cmd, shell=True,
                                  stdout=tornado.process.Subprocess.STREAM)
   p.set_exit_callback(partial(_command_done, name, callback, p))
 

setup.py

@@ -8,7 +8,7 @@ setup(
   name = 'nvchecker',
   version = nvchecker.__version__,
   packages = find_packages(),
-  install_requires = ['tornado', 'setuptools'],
+  install_requires = ['tornado>=4.1', 'setuptools'],
   tests_require=[
     'pytest',
   ],