Upgrade fs2-reactive-streams 0.6.0

[juanpedromoreno]

fs2-reactive-streams 0.6.0 is out. We could upgrade adapting our current codebase to the changes that the latest versions of fs2 and cats-effect are bringing.

[pepegar]

code to reproduce the bincompat error:

import monix.eval.Task
implicit val scheduler = monix.execution.Scheduler.Implicits.global
import cats.effect.IO
implicit val timer = scheduler.timer[IO]
Task(3).toIO

java.lang.NoSuchMethodError: cats.effect.IO$.ioConcurrentEffect()Lcats/effect/ConcurrentEffect;
  at monix.eval.Task.toIO(Task.scala:1622)
  ... 36 elided

[juanpedromoreno]

Let's wait for monix/monix#681

[juanpedromoreno]

Fixed by #472