upgrade to sbt-cross-building 0.8.0
upgrade build to sbt 0.12.4
next version
support building sbt-cross-building for sbt 0.13, fixes #23

Needed cross compatibility changes:
 * jline initializeTerminal
 * allDependenciesSettings change in sbt 0.13
 * ScopedKey in sbt 0.13 coming from Def.ScopedKey
 * specific specs2 versions per scala version