In my case it worked with the following build.sbt:
externalIvyFile()
classpathConfiguration in Compile := Compile
classpathConfiguration in Test := Test
classpathConfiguration in Runtime := Runtime
You just need the extra three lines in the end. Here is a link for more info: http://www.scala-sbt.org/release/docs/Detailed-Topics/Library-Management.html#ivy-file-dependency-configuration
Look for the Full Ivy Example. I hope it helps!
EDIT: Just to be complete - here is what pointed me to the above link: https://github.com/sbt/sbt/issues/849.