We’re still working on this. This is the most recent update:
"It’s a priority for us to be able to support Mojave in the near term. It will require a significant upgrade to our virtualization infrastructure to be able to run it: Mojave isn’t supported on the VMware version we are already running.
We’re working to make that happen in the next couple of weeks, after which we will be in a position to try to build new images that run Mojave."
I just got hit by this issue. This is actually a showstopper for anyone who wants to use Travis CI to deploy and build apps, as Xcode 10 does not support building dark mode apps in macOS 10.13. It would build, but spit out some innocuous messages in the build log that says “notice: Varying images and colors by appearance requires building on macOS 10.14 or later”, which means your apps will now look weird after using Travis to build. See https://stackoverflow.com/questions/52320727/building-macos-dark-mode-app-on-10-13-6-xcode-10
Checking to see if there is an ETA on Travis for MacOS 10.14
+1 this is critical for certain Mojave features.
Hey everyone,
Sorry for the lack of updates on this! I’ll see where we stand on things and share the update ASAP. 
Hey all,
Update here: We’re currently upgrading the virtualization infrastructure, which will allow us to run Mojave. After that, a few steps do still remain, but the majority of work will be complete. This is a very high priority for the Mac infrastructure team, so hope to have some more news real soon. 
There’s no timeline we can share right now, but it is close™!
Hey all!