Fix some bugs with report aggregation

A couple fixes to my previous pull request https://github.com/sbt/jacoco4sbt/pull/37:

1. executionDataFile now specifies the configuration
2. The task to generate a project's aggregate report now depends on the "cover" task of all its sub-projects. Without this, sbt could try to parallelize as much as possible and there would be no guarantee that the sub-projects would finish finish.