Restore compatibility with Gradle 2.0

Gradle 2.0, and only 2.0, requires a Plugin implementation to be
public. The changes made in gh-4113 (9c14ed3) made the class
package-private.

Closes gh-4139

Author: wilkinsona

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

@@ -36,7 +36,7 @@
  * @author Phillip Webb
  * @author Dave Syer
  */
-class SpringBootPlugin implements Plugin<Project> {
+public class SpringBootPlugin implements Plugin<Project> {
 
 	@Override
 	public void apply(Project project) {