Fixes#1466 Ref #2786
Even after fixing the mediator issue, we still have spurious binary
version conflict warning that does not account for sandbox
configurations.
This change follows the scalaVersionConfigs work.
Fixes#2786. Ref #2634.
sbt 0.13.12 added Ivy mediator that enforces scalaOrganization and
scalaVersion for Scala toolchain artifacts.
This turns out to be a bit too aggressive because Ivy configurations
can be used as an independent dependency graph that does not rely on
the scalaVersion used by Compile configuration. By enforcing
scalaVersion in those graph causes runtime failure.
This change checks if the configuration extends Default, Compile,
Provided, or Optional before enforcing scalaVersion.