2.13 regression: needs update for forkjoin removal

[SethTisue]

since scala/scala#6319 was merged, in the 2.13 community build we see:

[scala-async] [error] /home/jenkins/workspace/scala-2.13.x-integrate-community-build/target-0.9.11/project-builds/scala-async-84e9cc9153c08a616cd7ea17bda6bb38f890303b/src/test/scala/scala/async/run/futures/FutureSpec.scala:38: object forkjoin is not a member of package concurrent
[scala-async] [error]       implicit val ec = scala.concurrent.ExecutionContext.fromExecutor(new scala.concurrent.forkjoin.ForkJoinPool(), {
[scala-async] [error]                                                                                             ^

[NthPortal]

Looks like I missed this in #188, oops