Remove work-arounds for Scala 2.11 problematic dependencies

The ff0fd6eec658502f276f89c46f4aba0e0e268ddc introduced some exclusions
that were necessary for getting sbt to resolve dependencies properly
against Scala 2.11.0-M7.

Scala 2.11.0-M8 fixed its dependency structure so we can get rid of those
exclusions now.