Upgrade Spring Boot to v1.5.13

This merge request upgrades Spring boot from v1.5.10 to v1.5.13. Local testing seems to indicate no problems during the upgrade, however I was not able to test SAML locally, so we should take care while deploying this:

Major reason to upgrade for me was that the logger level of stale websocket connections has been decreased from LEVEL_ERROR to LEVEL_INFO. This makes sense, as stale connections were caused by browsers leaving without properly closing the websocket. Spring's SubProtocolWebSocketHandler takes care of that in it's next heartbeat task (where the error was logged in the first place), so it's safe to assume no serious problem is here. More info here

In short, this means we can resolve all "No messages received after XXX ms" exceptions in Errbit, and no new ones should pop up!