Remove compile-scoped junit dependency.

This avoids having junit classes showing up in the assembly jar.
I verified that only test classes in the jtransforms package
use junit.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #794 from vanzin/junit-dep-exclusion and squashes the following commits:

274e1c2 [Marcelo Vanzin] Remove junit from assembly in sbt build also.
ad950be [Marcelo Vanzin] Remove compile-scoped junit dependency.