Fixes #1439. Fixes metabuild downloading unused Scala 2.10.2

Fixes #1439 

Scala instance is added to the Ivy graph via autoLibraryDependency. For metabuilds, scala-library is scoped under “provided” configuration, which does not seem to evict modules on “compiled” configuration.

This PR turns `overrideScalaVersion` flag to `true` for the metabuilds, so override rules are added for the following modules:

- scala-library
- scala-compiler
- scala-reflect

/review @havocp 
This area of code predates the current modularization trend that's happening around Scala compiler and standard libraries. The first two come from [ArtifactInfo.java](https://github.com/sbt/sbt/blob/0.13/interface/src/main/java/xsbti/ArtifactInfo.java), which is used to share code between the launcher and sbt itself. I don't think the launcher needs to be aware of the "scala-reflect." Also I'm not sure if touching the interface is going to inadvertently create sbt 0.13.6 launcher dependence.