update to scala 2.13

Author: Cristian Popovici

.travis.yml

@@ -37,10 +37,6 @@ matrix:
     - jdk: openjdk11
       scala: 2.13.1
       env: COMMAND=ci-jvm
-    # Scala 2.13, JavaScript
-    - jdk: oraclejdk8
-      scala: 2.13.1
-      env: COMMAND=ci-js
 
 jdk:
   - oraclejdk8

build.sbt

@@ -8,13 +8,6 @@ version := "0.3.0"
 
 scalaVersion := "2.13.1"
 
-// Force building with Java 8
-initialize := {
-  val required = "1.8"
-  val current  = sys.props("java.specification.version")
-  assert(current == required, s"Unsupported build JDK: java.specification.version $current != $required")
-}
-
 scalacOptions ++= Seq(
   // warnings
   "-unchecked", // able additional warnings where generated code depends on assumptions