Filter out None scopes, to avoid trouble later.

I'm not sure why we have tasks with no set scope, but we've seen
this with custom tasks at Foursqare. Will figure that out later.

Testing Done:
Running ci.

Reviewed at https://rbcommons.com/s/twitter/r/1125/
Sync of https://github.com/pantsbuild/pants/commit/64c90727b2425a49894c7d5de35da54d6038f03f