Fix D test suite and add it to Travis CI

We require GNU make, so you can safely rely on `$(MAKE) -C $(VERSION_DIR)` being supported.

Otherwise, `cd $(VERSION_DIR) && $(MAKE)` is better than `cd $(VERSION_DIR) ; $(MAKE)` - if the cd fails (e.g. if there's already a file called `$(VERSION_DIR)` the second version will fail to change directory and then go on to run the make command in the current directory instead.