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-hypermedia/src/test/java/sample/hypermedia/SampleHypermediaApplicationHomePageTests.java

@@ -36,7 +36,7 @@ import org.springframework.test.context.web.WebAppConfiguration;
 import static org.junit.Assert.assertTrue;
 
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = SampleHypermediaApplication.class)
+@SpringApplicationConfiguration(SampleHypermediaApplication.class)
 @WebAppConfiguration
 @IntegrationTest("server.port=0")
 public class SampleHypermediaApplicationHomePageTests {