[Mac] SikuliLibrary keep throwing error on macOS

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### Environment
**OS:** macOS Mojave
**Python:** Python 3.7.0 / virtualenv
**JDK:** JDK8 update181
**SikuliLibrary:** Latest source code from GitHub, built with maven on macOS Mojave, run setup.py to install

### Description
The code of Robot Framework:
```
***Settings***
Library SikuliLibrary mode=NEW
Suite Setup Start Sikuli Process
Suite Teardown Stop Remote Server

***Test Cases***
Test
Add Image Path ${CURDIR}/img
Click test.png
```

On macOS, when run keywords to do some actions(e.g. Click), SikuliLibrary kept throwing error in Robot Framework test result:
```Connection to remote server broken: Remote end closed connection without response```

and I checked the SikuliLibrary's log:
```
0 [main] INFO org.robotframework.remoteserver.RemoteServer - Mapped path / to library com.github.rainmanwy.robotframework.sikulilib.SikuliLibrary.
1 [main] INFO org.robotframework.remoteserver.RemoteServer - Robot Framework remote server starting
1 [main] INFO org.eclipse.jetty.server.Server - jetty-7.x.y-SNAPSHOT
17 [main] INFO org.eclipse.jetty.server.handler.ContextHandler - started o.e.j.s.ServletContextHandler{/,null}
68 [main] INFO org.eclipse.jetty.server.AbstractConnector - Started SelectChannelConnector@0.0.0.0:54114
68 [main] INFO org.robotframework.remoteserver.RemoteServer - Robot Framework remote server started on port 54114.
3828 [qtp344560770-11] WARN org.eclipse.jetty.servlet.ServletHandler - /
org.apache.xmlrpc.XmlRpcException: I/O error while processing request: null
at org.apache.xmlrpc.server.XmlRpcStreamServer.execute(XmlRpcStreamServer.java:247)
at org.apache.xmlrpc.webserver.XmlRpcServletServer.execute(XmlRpcServletServer.java:112)
at org.apache.xmlrpc.webserver.XmlRpcServlet.doPost(XmlRpcServlet.java:196)
at org.robotframework.remoteserver.servlet.RemoteServerServlet.doPost(RemoteServerServlet.java:122)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.robotframework.remoteserver.servlet.RemoteServerServlet.service(RemoteServerServlet.java:102)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:565)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:479)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1031)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:406)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:965)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:117)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:111)
at org.eclipse.jetty.server.Server.handle(Server.java:348)
at org.eclipse.jetty.server.AbstractHttpConnection.handleRequest(AbstractHttpConnection.java:452)
at org.eclipse.jetty.server.AbstractHttpConnection.content(AbstractHttpConnection.java:894)
at org.eclipse.jetty.server.AbstractHttpConnection$RequestHandler.content(AbstractHttpConnection.java:948)
at org.eclipse.jetty.http.HttpParser.parseNext(HttpParser.java:851)
at org.eclipse.jetty.http.HttpParser.parseAvailable(HttpParser.java:235)
at org.eclipse.jetty.server.AsyncHttpConnection.handle(AsyncHttpConnection.java:77)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint.handle(SelectChannelEndPoint.java:606)
at org.eclipse.jetty.io.nio.SelectChannelEndPoint$1.run(SelectChannelEndPoint.java:46)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:603)
at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:538)
at java.lang.Thread.run(Thread.java:748)

```
Full log:
[Sikuli_java_stdout_1540398302.533548.txt](https://github.com/rainmanwy/robotframework-SikuliLibrary/files/2511613/Sikuli_java_stdout_1540398302.533548.txt)

Start by reproducing the Robot Framework example with SikuliLibrary in mode=NEW on macOS Mojave, using the Maven-built Java code installed through setup.py. Inspect the attached Sikuli Java stdout log around the Click request and remote server connection; done means the action completes without the remote server closing the connection.

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