Fix incremental compiler not being able to find JDK classes when compiler macros with Java 11, close #502

Motivation:

Jigsaw has changed the behavior of ClassLoaders.
For some reason, ScalaCompilerLoader with a null parent is able to find JDK classes on Java 8 but not on Java 11.

Modification:

Make the parent the root loader.

Result:

We can compile macros with Java 11.

Author: slandelle

src/main/java/scala_maven/ScalaCompilerLoader.java

@@ -16,6 +16,8 @@
 
 package scala_maven;
 
+import sbt.internal.inc.classpath.ClasspathUtil;
+
 import java.net.URL;
 import java.net.URLClassLoader;
 
@@ -25,7 +27,7 @@ public class ScalaCompilerLoader extends URLClassLoader {
   private final ClassLoader sbtLoader;
 
   public ScalaCompilerLoader(URL[] urls, ClassLoader sbtLoader) {
-    super(urls, null);
+    super(urls, ClasspathUtil.rootLoader());
     this.sbtLoader = sbtLoader;
   }