add Scala 2.13.0-Mx to `crossScalaVersions`

[SethTisue]

for x=1, currently

because of the circular dependency between scalac and scala-xml, the Scala release process also publishes scala-xml, so 1.0.6 is already published for 2.13.0-M1

so, this isn't pressing.

nonetheless, it would be good to have 2.13.0-M1 in crossScalaVersions, so that if a new scala-xml version is released, it gets published for that Scala version

[ashawley]

Ok, then we'll need definitely need to bump a la #141 to support a 2.13. Otherwise, mima will fail when building because it won't have a previous 2.13 build (unless somebody publishes a 1.0.5 build with 2.13). In this case, the circular dependency helps rather than hurts in this way.

[SethTisue]

Oh haha you already thought through what I just got done writing on the other ticket.

[ashawley]

Hilarious! Well, hopefully we came around to the same conclusions.

[ashawley]

Looks like @lrytz is working on this in #145

[ashawley]

The sbt-scala-modules plugin uses scalaVersionsByJvm to set `crossScalaVersions in the build, and it's done so for 2.13.0-M1