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-secure-oauth2/src/test/java/sample/secure/oauth2/SampleSecureOAuth2ApplicationTests.java

@@ -41,7 +41,7 @@ import static org.springframework.test.web.servlet.setup.MockMvcBuilders.webAppC
  */
 @RunWith(SpringJUnit4ClassRunner.class)
 @WebAppConfiguration
-@SpringApplicationConfiguration(classes = SampleSecureOAuth2Application.class)
+@SpringApplicationConfiguration(SampleSecureOAuth2Application.class)
 @IntegrationTest("server.port:0")
 public class SampleSecureOAuth2ApplicationTests {