Add Scala 2.13.0-RC1 support

[martijnhoekstra]

Like #234, but newer, and more modern.

lightbend-labs/scala-logging#159 is probably an upstream blocker.

[gslowikowski]

There are some serious problems with this version of Scala.

[SethTisue]

There are some serious problems with this version of Scala

can you elaborate?

[gslowikowski]

@SethTisue here you are:

• Mockito Scala (the same problem exists in ScalaTest)
  Problem with Scala 2.13.0-RC1 mockito/mockito-scala#95

• Scala Logging
  Ambiguous reference to overloaded definition in scala 2.13.0-RC1 lightbend-labs/scala-logging#157

Fortunately I managed to remove both dependencies (mockito and scala-logging).

[gslowikowski]

@SethTisue, I've created scala/make-release-notes#317