Add extra @Conditional to be defensive in Social autoconfig

Older versions of Spring Social will not have the SocialConfigurerAdapter
so making social autoconfig conditional on that class makes sense (since it
all extends from it).

Fixes gh-986

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

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.autoconfigure.social;
 
+import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.bind.RelaxedPropertyResolver;
 import org.springframework.context.EnvironmentAware;
 import org.springframework.core.env.Environment;
@@ -30,6 +31,7 @@ import org.springframework.social.connect.ConnectionFactory;
  * @author Craig Walls
  * @since 1.1.0
  */
+@ConditionalOnClass(SocialConfigurerAdapter.class)
 abstract class SocialAutoConfigurerAdapter extends SocialConfigurerAdapter implements
 		EnvironmentAware {
 

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

@@ -42,7 +42,7 @@ import org.springframework.web.servlet.view.BeanNameViewResolver;
  * @since 1.1.0
  */
 @Configuration
-@ConditionalOnClass({ ConnectController.class })
+@ConditionalOnClass({ ConnectController.class, SocialConfigurerAdapter.class })
 @AutoConfigureAfter(WebMvcAutoConfiguration.class)
 public class SocialWebAutoConfiguration {