Drop support for sbt 0.13.x.

It is not supported by sbt-scalajs 1.x nor by the latest master of
sbt-scala-native, so there is no point supporting it in
sbt-crossproject.

.travis.yml

@@ -26,7 +26,6 @@ script:
   - java -version
   - scripts/scalafmt --test
   - sbt sbt-crossproject-test/scripted
-  - sbt ++2.10.7 ^^0.13.17 sbt-crossproject-test/scripted
 
 cache:
   directories:

build.sbt

@@ -2,8 +2,7 @@ import Extra._
 
 inThisBuild(
   Def.settings(
-    crossSbtVersions := Seq((sbtVersion in Global).value, "0.13.17"),
-    crossScalaVersions := Seq("2.12.10", "2.10.7")
+    scalaVersion := "2.12.10"
   ))
 
 lazy val `sbt-crossproject-root` =

project/Extra.scala

@@ -20,11 +20,11 @@ object Extra {
   )
 
   // to publish plugin (we only need to do this once, it's already done!)
-  // follow: http://www.scala-sbt.org/0.13/docs/Bintray-For-Plugins.html
+  // follow: https://www.scala-sbt.org/1.x/docs/Bintray-For-Plugins.html
   // then add a new package ()
   // name: sbt-crossproject, license: BSD-like, version control: [email protected]:portable-scala/sbt-crossproject.git
   // to be available without a resolver
-  // follow: http://www.scala-sbt.org/0.13/docs/Bintray-For-Plugins.html#Linking+your+package+to+the+sbt+organization
+  // follow: http://www.scala-sbt.org/1.x/docs/Bintray-For-Plugins.html#Linking+your+package+to+the+sbt+organization
   lazy val publishSettings = Seq(
     publishArtifact in Compile := true,
     publishArtifact in Test := false,

sbt-crossproject/src/main/scala/sbtcrossproject/CrossProject.scala

@@ -11,7 +11,7 @@ final class CrossProject private[sbtcrossproject] (
     private val id: String,
     crossType: CrossType,
     val projects: Map[Platform, Project]
-) extends SBTCompat.CompositeProject {
+) extends CompositeProject {
 
   // CompositeProject API
   override def componentProjects: Seq[Project] = projects.valuesIterator.toSeq