Added error when mediawikiHost is not set, and used test (not example) settings for Travis

This would be better put into gulp, but as it is the settings replacement is in build, and we don't distinguish between travis builds, dev builds, and prod builds in gulp. Maybe someone else can refactor gulp tasks to do this. Anyway, as it is *localSettings.example.ts* gets used when `npm run-script dev` gets run the first time, and then node gives an error if mediawikiHost is still `null`. But on Travis, *localSettings.test.ts* gets used because the *local-settings-setup.sh* script gets run before build.