Improve conditions on FreeMarkerWebConfiguration

This commit makes sure to prevent the Freemarker web auto-configuration
to kick in if `spring-webmvc` is not on the classpath.

Closes gh-10027

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/freemarker/FreeMarkerAutoConfiguration.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.
@@ -129,7 +129,7 @@ public class FreeMarkerAutoConfiguration {
 	}
 
 	@Configuration
-	@ConditionalOnClass(Servlet.class)
+	@ConditionalOnClass({ Servlet.class, FreeMarkerConfigurer.class })
 	@ConditionalOnWebApplication
 	public static class FreeMarkerWebConfiguration extends FreeMarkerConfiguration {