Fix warning due to source being removed twice

[andrew-sayers]

Every time mate-screensaver is displayed, I get a message like Source ID N was not found when attempting to remove it. Source IDs are reused over time, so this could potentially lead to a more serious issue in some cases.

This silences that warning, and removes the risk of the more serious issue.

[lukefromdc]

I pushed a fix for master for some of the older travis build errors. On that commit the Fedora build finished but the deploy section failed again with

Install deployment dependencies
Authenticate deployment
no implicit conversion of nil into String (TypeError)
failed to deploy

which I do not know how to fix