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-devtools/src/test/java/org/springframework/boot/devtools/integrationtest/HttpTunnelIntegrationTest.java

@@ -61,7 +61,7 @@ import static org.junit.Assert.assertEquals;
  * @author Phillip Webb
  */
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = HttpTunnelIntegrationTest.Config.class)
+@SpringApplicationConfiguration(HttpTunnelIntegrationTest.Config.class)
 @WebIntegrationTest
 public class HttpTunnelIntegrationTest {