Update to gitbase 0.20 beta

[carlosms]

gitbase 0.20 is in beta. We can start to work integrating it in a new feature branch, gitbase-v0.20.0.

The breaking changes should not affect us. But it now includes a mysql client in the docker image, which means we can replace the new mysql image introduced in Engine v0.12.0, and use the gitbase image only.