Add fetch_default_bridges.sh to CI

We have scripts/fetch_default_bridges.sh which, according to the README.md during release, we have to be sure to run this in order to get the updated bridges.

Is there any reason why this cannot just be added to the assembleDebug job to be run in the CI? It would run more often then we release, but its just a simple curl call to our own repositories, so I don't see any harm in doing so. If we did this, then we could remove a manual step that we have to be sure not to forget on release, because it would be automated.