py37 (#1256)

* add py37 to travisci

* use dist:xenial for py37

* sudo: true in .travici

* bump websockets version for py37 support and fix unit tests

tests/test_worker.py

@@ -131,5 +131,5 @@ def test_worker_close(worker):
     loop.run_until_complete(_close)
 
     assert worker.signal.stopped == True
-    conn.websocket.close_connection.assert_called_with(after_handshake=False)
+    assert conn.websocket.close_connection.called == True
     assert len(worker.servers) == 0