Let Travis print GHC/Cabal versions before build
Fix wrong doc: millisecond -> microsecond
Refactor restarter a bit; still buggy.

The restarter isn't triggered until a minimum amount of nodes have been
spawned. However, once triggered, it is triggered periodically,
regardless of whether there are actually new nodes connecting or not.
Make use of new Cabal 1.20's exec parameter

Use 'cabal exec' instead of manually making GHC(i) sandbox-aware. The
previous implementation was functionally identical, but much hackier.