Fixes #1439. Fixes metabuild downloading unused Scala 2.10.2

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 commit turns overrideScalaVersion flag to true for the metabuilds,
so override rules are added for the following modules:
- scala-library
- scala-compiler
- scala-reflect
Merge pull request #1538 from sbt/fix/1439

Fixes #1439. Fixes metabuild downloading unused Scala 2.10.2