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-testng/src/test/java/sample/testng/SampleTestNGApplicationTests.java

@@ -34,7 +34,7 @@ import static org.testng.Assert.assertEquals;
  *
  * @author Phillip Webb
  */
-@SpringApplicationConfiguration(classes = SampleTestNGApplication.class)
+@SpringApplicationConfiguration(SampleTestNGApplication.class)
 @WebIntegrationTest("server.port:0")
 @DirtiesContext
 public class SampleTestNGApplicationTests extends AbstractTestNGSpringContextTests {