Test the Gradle plugin against Gradle 4.8

Closes gh-13359

Author: wilkinsona

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/junit/GradleCompatibilitySuite.java

@@ -39,7 +39,7 @@
 public final class GradleCompatibilitySuite extends Suite {
 
 	private static final List<String> GRADLE_VERSIONS = Arrays.asList("default", "4.1",
-			"4.2", "4.3", "4.4.1", "4.5.1", "4.6", "4.7");
+			"4.2", "4.3", "4.4.1", "4.5.1", "4.6", "4.7", "4.8");
 
 	public GradleCompatibilitySuite(Class<?> clazz) throws InitializationError {
 		super(clazz, createRunners(clazz));