Dropped covariance annotation on Lub (see https://github.com/milessabin/shapeless/issues/242)
Changes in coproduct Unifier. Fixes https://github.com/milessabin/shapeless/issues/242
Merge pull request #243 from alexarchambault/coproductunify

Coproduct unify (https://github.com/milessabin/shapeless/issues/242)