Cross build with Scala 2.10.4

Enable SBT cross build [1]

To build against all versions, prefix the action to run with `+` like:

`> + publish`

[1] http://www.scala-sbt.org/0.13.2/docs/Detailed-Topics/Cross-Build.html
Set default Scala version to 2.10.4
Merge pull request #80 from guersam/tmp/cross-build

Cross build with Scala 2.10.4