title: SBT cross building - choosing a different library version for different scala version
body: <p>Is it possible to configure SBT to use a completely different library version when cross building, depending on the scala version the project is being built with?</p>
  
  <p>For example:</p>
  
  <ul>
  <li>When building with Scala <code>2.9.2</code> I want to use <code>"org.scalatest" % "scalatest_2.9.2" % "2.0.M5" % "test"</code></li>
  <li>When building with scala <code>2.10.0</code> I want to use <code>"org.scalatest" % "scalatest_2.10.0-RC5" % "2.0.M5-B1" % "test"</code></li>
  </ul>