Only auto-configure SpringSocialDialect for Thymeleaf 2

Previously, SocialWebAutoConfiguration would create a
SpringSocialDialect bean when SpringTemplateEngine was on the
classpath. This class exists in both Thymeleaf 2 and Thymeleaf 3 but
SpringSocialDialect is only compatible with Thymeleaf 2.

This commit updates the auto-configuration to require
SpringResourceResourceResolver to be on the classpath. This class
exists in Thymeleaf 2 but does not exist in Thymeleaf 3.

Closes gh-4858

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/social/SocialWebAutoConfiguration.java

 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,7 +18,7 @@ package org.springframework.boot.autoconfigure.social;
 
 import java.util.List;
 
-import org.thymeleaf.spring4.SpringTemplateEngine;
+import org.thymeleaf.spring4.resourceresolver.SpringResourceResourceResolver;
 
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.AutoConfigureAfter;
@@ -164,7 +164,7 @@ public class SocialWebAutoConfiguration {
 	}
 
 	@Configuration
-	@ConditionalOnClass(SpringTemplateEngine.class)
+	@ConditionalOnClass(SpringResourceResourceResolver.class)
 	protected static class SpringSocialThymeleafConfig {
 
 		@Bean