Correct minimum recommended version of Gradle to be 2.9 rather than 2.8

Closes gh-7189
See gh-6884

spring-boot-docs/src/main/asciidoc/getting-started.adoc

@@ -40,7 +40,7 @@ By default, Spring Boot {spring-boot-version} requires http://www.java.com[Java
 Spring Framework {spring-version} or above. You can use Spring Boot with Java 6 with some
 additional configuration. See <<howto.adoc#howto-use-java-6>> for more details. Explicit
 build support is provided for Maven (3.2+) and Gradle (1.12 or 2.x). Support for Gradle
-2.7 and earlier is deprecated. Gradle 3 is not supported.
+2.8 and earlier is deprecated. Gradle 3 is not supported.
 
 TIP: Although you can use Spring Boot with Java 6 or 7, we generally recommend Java 8 if
 at all possible.
@@ -205,7 +205,7 @@ scope.
 
 [[getting-started-gradle-installation]]
 ==== Gradle installation
-Spring Boot is compatible with Gradle 1.12 or 2.x but support for 2.7 and earlier is
+Spring Boot is compatible with Gradle 1.12 or 2.x but support for 2.8 and earlier is
 deprecated. Gradle 2.14.1 is recommended. Gradle 3 is not supported. If you don't already
 have Gradle installed you can follow the instructions at http://www.gradle.org/.
 

spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/SpringBootPlugin.java

@@ -45,10 +45,10 @@ public class SpringBootPlugin implements Plugin<Project> {
 
 	@Override
 	public void apply(Project project) {
-		if (GradleVersion.current().compareTo(GradleVersion.version("2.8")) < 0) {
+		if (GradleVersion.current().compareTo(GradleVersion.version("2.9")) < 0) {
 			logger.warn("Spring Boot plugin's support for Gradle "
 					+ GradleVersion.current().getVersion()
-					+ " is deprecated. Please upgrade to Gradle 2.8 or later.");
+					+ " is deprecated. Please upgrade to Gradle 2.9 or later.");
 		}
 		project.getExtensions().create("springBoot", SpringBootPluginExtension.class,
 				project);