Use scala 2.13.0-M4 milestone release

Author: ashawley

Diff for: README.md

@@ -2,7 +2,7 @@ scala-xml
 [![Travis](https://img.shields.io/travis/scala/scala-xml.svg)](https://travis-ci.org/scala/scala-xml)
 [![latest release for 2.11](https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-xml_2.11.svg?label=scala+2.11)](http://mvnrepository.com/artifact/org.scala-lang.modules/scala-xml_2.11)
 [![latest release for 2.12](https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-xml_2.12.svg?label=scala+2.12)](http://mvnrepository.com/artifact/org.scala-lang.modules/scala-xml_2.12)
-[![latest release for 2.13.0-M2](https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-xml_2.13.0-M2.svg?label=scala+2.13.0-M2)](http://mvnrepository.com/artifact/org.scala-lang.modules/scala-xml_2.13.0-M2)
+[![latest release for 2.13.0-M4](https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-xml_2.13.0-M4.svg?label=scala+2.13.0-M4)](http://mvnrepository.com/artifact/org.scala-lang.modules/scala-xml_2.13.0-M4)
 [![Gitter](https://badges.gitter.im/Join+Chat.svg)](https://gitter.im/scala/scala-xml)
 =========
 

Diff for: build.sbt

@@ -3,7 +3,7 @@ import ScalaModulePlugin._
 scalaVersionsByJvm in ThisBuild := {
   val v211 = "2.11.12"
   val v212 = "2.12.4"
-  val v213 = "2.13.0-M3"
+  val v213 = "2.13.0-M4"
   Map(
     6 -> List(v211 -> true),
     7 -> List(v211 -> false),
@@ -31,7 +31,7 @@ lazy val xml = crossProject.in(file("."))
       (unmanagedSourceDirectories in Compile).value.map { dir =>
         val sv = scalaVersion.value
         CrossVersion.partialVersion(sv) match {
-          case Some((2, 13)) if !sv.startsWith("2.13.0-M3") => file(dir.getPath ++ "-2.13") // TODO: remove M3 guard once M4 is out.
+          case Some((2, 13)) => file(dir.getPath ++ "-2.13")
           case _             => file(dir.getPath ++ "-2.11-2.12")
         }
       }