During the, about to be finished, Alpine City sprint I made some changes that I already had sketched for quite a while:
Trigger jenkins jobs from within github with a simple comment.
And that's exactly what one can do since yesterday 
See for example:
After creating a pull request, mr.roboto will add a comment with the instructions on how to trigger them.
The reason for not automatically triggering jenkins jobs (i.e. like travis and such services) is that as the tests do take quite a while and there are quite a few jobs it would be a waste of resources keep running jobs on every push on a branch.
There are also contributors that create WIP (work in progress) pull requests, which would be pointless to run tests against a still moving target.
I hope this will lower the barrier to contribute to Plone and will make everyone more productive 
If you find any problem with it, or any improvement, please don't hesitate to report it either here or on https://github.com/plone/jenkins.plone.org