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-data-jpa/src/test/java/sample/data/jpa/SampleDataJpaApplicationTests.java

@@ -45,7 +45,7 @@ import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.
  * @author Dave Syer
  */
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = SampleDataJpaApplication.class)
+@SpringApplicationConfiguration(SampleDataJpaApplication.class)
 @WebAppConfiguration
 // Enable JMX so we can test the MBeans (you can't do this in a properties file)
 @TestPropertySource(properties = { "spring.jmx.enabled:true",

spring-boot-samples/spring-boot-sample-data-jpa/src/test/java/sample/data/jpa/service/CityRepositoryIntegrationTests.java

@@ -36,7 +36,7 @@ import static org.junit.Assert.assertThat;
  * @author Oliver Gierke
  */
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = SampleDataJpaApplication.class)
+@SpringApplicationConfiguration(SampleDataJpaApplication.class)
 public class CityRepositoryIntegrationTests {
 
 	@Autowired

spring-boot-samples/spring-boot-sample-data-jpa/src/test/java/sample/data/jpa/service/HotelRepositoryIntegrationTests.java

@@ -44,7 +44,7 @@ import static org.junit.Assert.assertThat;
  * @author Oliver Gierke
  */
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = SampleDataJpaApplication.class)
+@SpringApplicationConfiguration(SampleDataJpaApplication.class)
 public class HotelRepositoryIntegrationTests {
 
 	@Autowired