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, this PR uses *localSettings.example.ts* 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. The reason why this is necessary is that *localSettings.example.ts* has no valid `mediawikiHost`, which is something that will be needed once we start making integration tests. Travis hasn't failed so far because we have no integration tests, but if it looked at the DOM it would notice that there is nothing but an error there.

@aquilax 
I would actually throw here, without mediawiki host, there is not much purpose for this app running