limit changes to scalaVersion and unmanagedSourceDirectories

## steps
1. create a multi project `build.scala` that uses sbt-cross-building
2. set the common setting defaulting `scalaVerson` to `"2.10.0"`

## problem
`scalaVersion` setting is ignored and it defaults to 2.9.2.

## expectation
non-plugin project defaults to `"2.10.0"` if the user said so.

## what this changes
this pull req checks for `sbtPlugin` and limits changes to `scalaVersion` and `unmanagedSourceDirectories`. 

## best practice
ideally, the plugin should *not* override the `settings` so I can manually load `sbt.CrossBuilding.settings` only to plugin project. this is the first recommendation in [Plugin Best Practice](http://www.scala-sbt.org/release/docs/Extending/Plugins-Best-Practices).