gradle: Improve clean task by using BndProject.clean()

This will then recreate the output folders after cleaning them. This will
solve the problem of using the clean task before other tasks in the same
gradle execution.

Based upon suggestion of https://github.com/bndtools/bnd/pull/555

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #565 from bjhargrave/gradle-clean

gradle: Improve clean task by using BndProject.clean()