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/src/test/java/sample/secure/SampleSecureApplicationTests.java

@@ -43,7 +43,7 @@ import static org.junit.Assert.assertEquals;
  * @author Dave Syer
  */
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = { SampleSecureApplication.class,
+@SpringApplicationConfiguration({ SampleSecureApplication.class,
 		TestConfiguration.class })
 public class SampleSecureApplicationTests {