drop Scala 2.13 from the cross-build

not needed anymore since partest we re-in-sourced into scala/scala

Author: SethTisue

build.sbt

@@ -7,13 +7,14 @@ name    := "scala-partest"
 version := "1.1.4-SNAPSHOT"
 
 scalaVersionsByJvm in ThisBuild := {
-  val vs = List("2.12.6", "2.13.0-M3")
+  val vs = List("2.12.6")
 
   Map(
     8 -> vs.map(_ -> true),
-    9 -> vs.map(_ -> false)),
-    10 -> vs.map(_ -> false)),
-    11 -> vs.map(_ -> false))
+    9 -> vs.map(_ -> false),
+    10 -> vs.map(_ -> false),
+    11 -> vs.map(_ -> false)
+  )
 }
 
 scalaXmlVersion := "1.0.6"