Error in example for the latest sbt version

[kioria]

The Task Graph page http://www.scala-sbt.org/1.x/docs/Task-Graph.html contains example for conditional task evaluation, namely this code snippet:

    scalacOptions := {
      val ur = update.value  // update task happens-before scalacOptions
      if (false) {
        val x = clean.value  // clean task happens-before scalacOptions
      }
      ur.allConfigurations.take(3)
    }

When running on the sbt version 1.0.2, build is failing with:

error: The evaluation of clean happens always inside a regular task.

So the example should be fixed to meet the newer sbt requirement.

[dwijnand]

Thanks. Related to #501.