rename PartestTask to SbtPartestTask

`PartestTask` is a very unfortunate name for this, because it already
exists in scala-partest; there is:

 - `scala.tools.partest.PartestTask extends org.apache.tools.ant.Task`
   in scala-partest, for running partest with ant
 - `scala.tools.partest.PartestTask extends sbt.testing.Task` for
   running partest with sbt

Depending on the order `scala-partest` and `scala-partest-interface` are
added to the dependencies, SBT can get the wrong one and crash...
bump versions
Merge pull request #4 from gourlaysama/wip/partest-task

rename PartestTask to SbtPartestTask and bump versions