DebugKit 3.15.0

• Require cakephp 3.6.0 min
• Don't use deprecated methods.
• DebugKit now attempts to prevent display in environments that look
  like non-local environments as we've had a number of reported issues of
  debugkit displaying in production environments by poorly configured
  systems. This can be overridden by using the configuration key Debugkit.forceEnable