fix duplicate mappings error when running test command

Indicating "resourceManaged" as output in purescript base setting
causes an error about "duplicate mappings" when running "test" in sbt.

The solution consist on using "crossTarget" instead of "resourceManaged".

Steps to reproduce it:

- clone this repo
- cd into play-example
- run sbt test

A similar situation has been depicted here:
https://github.com/softprops/coffeescripted-sbt/issues/6

closes #14