Just leave github issue tracker as default

review @stevecrozz

the idea behind the extraction of services into plugins was that we dont need to include all not necessary gems.

We should include one plugin when you install errbit out-of-the-box. I think having the github issue tracker as the default is the best choice.

Also having all those plugins as hard dependencies means that every change we need to merge it back/own all the other plugins, which does not scale at all.