Do not run RxScalaDemo on each build

On my last pull request, I forgot to uncomment the line which ignores `RxScalaDemo`, so currently it is executed on each build, even though it does no automatic testing. And since `RxScalaDemo` takes about 48 seconds, this significantly increases build time. Sorry for this...

After this change, the complete build took 3 mins 33.932 secs on my 3 years old laptop, so if cloudbees can't build it in 10 min, there might be something else not properly working...