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

ivy/src/main/scala/sbt/IvyScala.scala

@@ -16,6 +16,7 @@ object ScalaArtifacts {
   val Organization = ScalaOrganization
   val LibraryID = ScalaLibraryID
   val CompilerID = ScalaCompilerID
+  val ReflectID = "scala-reflect"
   def libraryDependency(version: String): ModuleID = ModuleID(Organization, LibraryID, version)
 
   private[sbt] def toolDependencies(org: String, version: String): Seq[ModuleID] = Seq(
@@ -47,6 +48,7 @@ private object IvyScala {
   def overrideScalaVersion(module: DefaultModuleDescriptor, version: String) {
     overrideVersion(module, Organization, LibraryID, version)
     overrideVersion(module, Organization, CompilerID, version)
+    overrideVersion(module, Organization, ReflectID, version)
   }
   def overrideVersion(module: DefaultModuleDescriptor, org: String, name: String, version: String) {
     val id = new ModuleId(org, name)