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-atmosphere/src/test/java/sample/atmosphere/SampleAtmosphereApplicationTests.java

@@ -47,7 +47,7 @@ import static org.hamcrest.Matchers.equalTo;
 import static org.junit.Assert.assertThat;
 
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = SampleAtmosphereApplication.class)
+@SpringApplicationConfiguration(SampleAtmosphereApplication.class)
 @WebIntegrationTest(randomPort = true)
 @DirtiesContext
 public class SampleAtmosphereApplicationTests {