Add value alias for SpringApplicationConfiguration

Given that Spring Boot uses java config accross the board, a new `value`
attribute is now aliased to the existing `classes` attribute such that
one could write the following:

@SpringApplicationConfiguration(MyConfig.class)
public class MyTest {}

Closes gh-3635

spring-boot-samples/spring-boot-sample-metrics-redis/src/test/java/sample/metrics/redis/SampleRedisExportApplicationTests.java

@@ -30,7 +30,7 @@ import org.springframework.test.context.web.WebAppConfiguration;
  * @author Dave Syer
  */
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = SampleRedisExportApplication.class)
+@SpringApplicationConfiguration(SampleRedisExportApplication.class)
 @WebAppConfiguration
 @IntegrationTest({ "server.port=0", "spring.jmx.enabled=true" })
 @DirtiesContext