Update JFunction code to latest version

[szeiger]

The JFunctions in scala-java8-compat are Serializable, the ones that were published in Scala 2.12.0-M4 are not. This should be fixed for M5.

[lrytz]

maybe, instead of JFunctionN, we should make scala's FunctionN serialziable, to solve the issue described in scala/scala-java8-compat#41 ?