Java 11 build problems #243

Unable to build using java 11.

• Old Mockito version is not working on java 11.
• Latest Mockito cannot mock IOException throw if interface cannot throw IOException.

Pull request contains fix for java 11 build.

• bump Mockito to 2.23.4
• fix unit test WebsocketConnectionTest.testIncomingMessageIOError

org.mockito.exceptions.base.MockitoException: Checked exception is invalid for this method! Invalid: java.io.IOException