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-cache/src/test/java/sample/cache/SampleCacheApplicationTests.java

@@ -33,7 +33,7 @@ import static org.hamcrest.Matchers.is;
 import static org.hamcrest.core.IsNull.notNullValue;
 
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = { SampleCacheApplication.class })
+@SpringApplicationConfiguration(SampleCacheApplication.class)
 public class SampleCacheApplicationTests {
 
 	@Autowired