unexpected keyword argument 'debug' error

Due to this change, https://bugs.launchpad.net/zaqar/+bug/1558062
new installs of robot_blockly result in the error in the issue title. 
The debug argument should be removed from [here](https://github.com/erlerobot/robot_blockly/blob/master/scripts/robot_blockly_backend.py#L391) and [here](https://github.com/erlerobot/robot_blockly/blob/master/scripts/server.py#L143)
