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-web-method-security/src/test/java/sample/security/method/SampleMethodSecurityApplicationTests.java

@@ -48,7 +48,7 @@ import static org.junit.Assert.assertTrue;
  * @author Dave Syer
  */
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = SampleMethodSecurityApplication.class)
+@SpringApplicationConfiguration(SampleMethodSecurityApplication.class)
 @WebAppConfiguration
 @IntegrationTest("server.port:0")
 @DirtiesContext