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-data-mongodb/src/test/java/sample/data/mongo/SampleMongoApplicationTests.java

@@ -33,7 +33,7 @@ import static org.junit.Assert.assertTrue;
  * @author Andy Wilkinson
  */
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = SampleMongoApplication.class)
+@SpringApplicationConfiguration(SampleMongoApplication.class)
 @IntegrationTest
 public class SampleMongoApplicationTests {