Update build and test system, add package deployment

• update gradle file
• publish gradle package
• use tests from gradle dependencies

buildSrc/src/main/java/org/jastadd/relast/plugin/RelastPlugin.java

-package org.jastadd.relast.plugin;
-
-import org.gradle.api.Plugin;
-import org.gradle.api.Project;
-import org.gradle.api.Task;
-import org.gradle.api.tasks.TaskCollection;
-
-import java.util.Set;
-
-/**
- * Plugin for RelAst-Test.
- *
- * @author rschoene - Initial contribution
- */
-public class RelastPlugin implements Plugin<Project> {
-
-  private Task testTask;
-
-  @Override
-  public void apply(Project project) {
-    Set<Task> tasks = project.getTasksByName("test", false);
-    // there should be only one task "test"
-    testTask = tasks.iterator().next();
-    TaskCollection<RelastTest> relastTests = project.getTasks().withType(RelastTest.class);
-    relastTests.forEach(this::setupRelastTest);
-    relastTests.whenTaskAdded(this::setupRelastTest);
-  }
-
-  private void setupRelastTest(RelastTest relastTest) {
-    testTask.dependsOn(relastTest);
-    relastTest.setGroup("verification");
-  }
-}