Update base.yaml kactivitymanagerd override

[ScarlettGatelyClark]

upstream only has a master branch and therefore fails looking for Plasma/5.5 this update creates an override pointing to the master branch.
I left neon packaging in the override as I assume they will be affected.

[apachelogger]

<sitter> sgclark: that change actually is wrong
<sitter> kactivitymanagerd has no stable branch
<sgclark> interesting
<sitter> only gets one with 5.6 I presume
<sgclark> I did not say stable branch. I said master
<sitter> yeah, you are mapping kubuntu_*stable* to upstream master
<sitter> which is conceptually wrong
<sgclark> oh.
<sgclark> I see the error of my ways now
<sitter> the problem with that is that it will later prevent kactivitymanagerd from using the common plasma stable branch btw, so that would cause head scratching