Build improvements - less repetition

Please review and merge.
We had distinguished between `specsSbt` and `specsBuild` because Play itself was on Scala 2.10 while SBT was on Scala 2.9.2, and specs had stopped cross building for Scala 2.9.2, meaning we had to have different versions for the SBT plugin and for Play.  When we upgraded to SBT 0.13, that meant SBT was now on 2.10 so we could change this so that they used the same version, but in future it might change again to a situation where we need different versions, so I'd rather keep the distinction.