updateSbtClassifiers broken with sbt 0.13.12 on Scala 2.9 projects

## steps

Create a simple project with this in `build.sbt`:

``` scala
scalaVersion := "2.9.3"
```

run `sbt -Dsbt.version=0.13.12 updateSbtClassifiers`
## problem

Task fails with resolve error:

```
[error] (*:updateSbtClassifiers) sbt.ResolveException: unresolved dependency: org.scala-lang#scala-reflect;2.9.3: not found
```
## notes

This problem does not occur up to sbt 0.13.11
