CI doesn't handle feature flags with workspace

[Nemo157]

Correctly testing feature flags in workspaces is hard. The current CI setup doesn't handle this appropriately. For some of the issues you could look at how complicated the futures workspaces .travis.yml is and its history.

I'll try and port some of the simpler ideas from the futures setup across here so that as more feature flags are added basic compatibility can be tested (actually exhaustively testing feature flags becomes impossible quickly because there are O(n!) combinations, but with just a few extra builds you can test for the most likely issues).