Fails to build on 2.12.0-RC1

[danslapman]

implicit method defaultExecutionContext is not applicable here because it comes after the application point and it lacks an explicit result type
https://travis-ci.org/scala/scala-continuations/jobs/159573627

[SethTisue]

the changes in #5141 and #5294 commonly cause errors like this on implicits that lack explicitly declared result types. declaring the type is good practice anyway

[danslapman]

https://travis-ci.org/scala/scala-continuations/jobs/159716395, can be related to scala/scala-dev#219, will continue my investigation tomorrow

[danslapman]

Fixed by #21