Update activity.php

Do not return every time, otherwise all triggers like onBeforePost will just work on the first plugin and not on the other.
This patch will not fix the compatibility with the existing plugins because some are returning "true" for generic triggers.
It is also possible to get all values and return a merge of these, but the __call function must call every activity plugins.
Update activity.php

Return the first value we got during the call of the instances.
(in order to keep the compatibility with the previous modification of the function, to use a trigger in order to get the number of points or medals)
Merge pull request #2580 from obsidev/patch-2

Activity Integration don't call every activity plugins