support for tornado 5

[Ferguzz]

tornado==5.0 with pytest-tornado==0.4.5, tests error with

    def _close():
        io_loop.clear_current()
>       if (not tornado.ioloop.IOLoop.initialized() or
                io_loop is not tornado.ioloop.IOLoop.instance()):
E               AttributeError: type object 'IOLoop' has no attribute 'initialized'

.tox/integration-test/lib/python3.6/site-packages/pytest_tornado/plugin.py:136: AttributeError

[vidartf]

There's a suggested fix in #30.

[Ferguzz]

@vidartf awesome thanks

[apast]

Thanks for all!! @vidartf and @eugeniy