Added 'buildScmUpdated' event

[CheapDevotion]

While developing plugins for node-ci, I needed the ability to edit build files after they were checked out, but before the steps were run. To address this, I added a 'buildScmUpdated' event that is called right after a scm update is performed.

[okv]

Hey there! Sorry for the late reply. The event itself looks good :+1: But I think it would be better to emit it from _emitBuildUpdate function (which is called from _updateBuild ). Could you also add some tests? (you might take a look at test/distributor/buildEvents.js)