Reconsider UPX as a config

As part of the analysis in https://github.com/zeit/now-builders/issues/63 we discovered that using UPX adds significant startup overhead to the launcher. It was done in this repo as part of #1.

We were only doing it to reduce the resulting lambda's size, which is probably no longer necessary as Now 2.0 has configurable sizes up to 50MB now.

The only time it would make sense for us to support an UPXed launcher is if a user explicitly opts in. Let's wait until somebody asks for that because it would probably add some complexity to this repo (would need 2 launchers).