Conditionally exclude the Position.scala file in Scala <2.11.

Position is something we imported from scala.io because it was removed in Scala
2.11; in 2.10, we can lean on the one in the scala library. For people who used
sbt assembly to build fat JARs, having both in two different JARs resulted in
nasty-sauce. We now exclude it when building for Scala versions below 2.11 so
this issue doesn't come up.

This fixes #1588.