Make compatible with latest gevent version (#583)

See https://github.com/gevent/gevent/blob/master/CHANGES.rst#13b2-2018-05-03
```
The long-deprecated and undocumented module gevent.wsgi was removed.
```

connexion/apps/flask_app.py

@@ -106,10 +106,10 @@ def run(self, port=None, server=None, debug=None, host=None, **options):  # prag
             tornado.ioloop.IOLoop.instance().start()
         elif self.server == 'gevent':
             try:
-                import gevent.wsgi
+                import gevent.pywsgi
             except ImportError:
                 raise Exception('gevent library not installed')
-            http_server = gevent.wsgi.WSGIServer((self.host, self.port), self.app, **options)
+            http_server = gevent.pywsgi.WSGIServer((self.host, self.port), self.app, **options)
             logger.info('Listening on %s:%s..', self.host, self.port)
             http_server.serve_forever()
         else: